feat: migrate c/comment from cppref [#44]

src/content/docs/c/comment.mdx

@@ -0,0 +1,156 @@
+---
+title: "Comments"
+---
+
+import { Desc, DescList, DocLink, Revision, RevisionBlock } from '@components/index';
+
+Comments serve as a sort of in-code documentation. When inserted into a program, they are effectively ignored by the compiler; they are solely intended to be used as notes by the humans that read source code.
+
+### Syntax
+
+* (1) `/*` _comment_ `*/`
+* <Revision since="C99">(2) `//` _comment_ </Revision>  
+
+1) Often known as "C-style" or "multi-line" comments.
+2) Often known as "C++-style" or "single-line" comments.
+
+All comments are removed from the program at <DocLink src="/c/language/translation_phases">translation phase 3</DocLink> by replacing each comment with a single whitespace character.
+
+### C-style
+
+C-style comments are usually used to comment large blocks of text or small fragments of code; however, they can be used to comment single lines. To insert text as a C-style comment, simply surround the text with `/*` and `*/`. C-style comments tell the compiler to ignore all content between `/*` and `*/`. Although it is not part of the C standard, `/**` and `**/` are often used to indicate documentation blocks; this is legal because the second asterisk is simply treated as part of the comment.
+
+Except within a <DocLink src="/c/language/character_constant">character constant</DocLink>, a <DocLink src="/c/language/string_literal">string literal</DocLink>, or a comment, the characters `/*` introduce a comment. The contents of such a comment are examined only to identify multibyte characters and to find the characters `*/` that terminate the comment. C-style comments cannot be nested.
+
+<RevisionBlock since="C99">
+
+### C++-style
+
+C++-style comments are usually used to comment single lines of text or code; however, they can be placed together to form multi-line comments. To insert text as a C++-style comment, simply precede the text with `//` and follow the text with the new line character. C++-style comments tell the compiler to ignore all content between `//` and a new line.
+
+Except within a <DocLink src="/c/language/character_constant">character constant</DocLink>, a <DocLink src="/c/language/string_literal">string literal</DocLink>, or a comment, the characters `//` introduce a comment that includes all multibyte characters up to, but not including, the next new-line character. The contents of such a comment are examined only to identify multibyte characters and to find the new-line character that terminates the comment. C++-style comments can be nested:
+
+```c
+//  y = f(x);   // invoke algorithm
+```
+
+A C-style comment may appear within a C++-style comment:
+
+```c
+//  y = f(x);   /* invoke algorithm */
+```
+
+A C++-style comment may appear within a C-style comment; this is a mechanism for excluding a small block of source code:
+
+```c
+/*
+    y = f(x);   // invoke algorithms
+    z = g(x);
+*/
+```
+
+</RevisionBlock>
+
+### Notes
+
+Because comments <DocLink src="/c/language/translation_phases">are removed</DocLink> before the preprocessor stage, a macro cannot be used to form a comment and an unterminated C-style comment doesn't spill over from an #include'd file.
+
+```c
+/* An attempt to use a macro to form a comment. */
+/* But, a space replaces characters "//".       */
+#ifndef DEBUG
+    #define PRINTF //
+#else
+    #define PRINTF printf
+#endif
+...  
+PRINTF("Error in file%s at line%i\n", __FILE__, __LINE__);
+```
+
+Besides commenting out, other mechanisms used for source code exclusion are:
+
+```c
+#if 0
+    puts("this will not be compiled");
+    /* no conflict with C-style comments */
+    // no conflict with C++-style comments
+#endif
+```
+
+and
+
+```c
+if(0) {
+    puts("this will be compiled but not be executed");
+    /* no conflict with C-style comments */
+    // no conflict with C++-style comments
+}
+```
+
+The introduction of // comments in C99 was a breaking change in some rare circumstances:
+
+```c
+a = b //*divisor:*/ c
++ d; /* C89 compiles a = b / c + d;
+        C99 compiles a = b + d; */
+```
+
+### Example
+
+```cpp
+#include <stdio.h>
+/*
+C-style comments can contain
+multiple lines.
+*/
+
+/* Or, just one line. */
+
+// C++-style comments can comment one line.
+
+// Or, they can
+// be strung together.
+
+int main(void)
+{
+  // The below code won't be run
+  // puts("Hello");
+
+  // The below code will be run
+  puts("World");
+
+  // A note regarding backslash + newline.
+  // Despite belonging to translation phase 2 (vs phase 3 for comments),
+  // '\' still determines which portion of the source code is considered
+  // as 'comments':
+  // This comment will be promoted to the next line \
+  puts("Won't be run"); // may issue a warning "multi-line comment"
+  puts("Hello, again");
+}
+```
+
+Output:
+
+```text
+World
+Hello, again
+```
+
+### References
+
+* C17 standard (ISO/IEC 9899:2018):
+  * 6.4.9 Comments (p: 54)
+* C11 standard (ISO/IEC 9899:2011):
+  * 6.4.9 Comments (p: 75)
+* C99 standard (ISO/IEC 9899:1999):
+  * 6.4.9 Comments (p: 66)
+* C89/C90 standard (ISO/IEC 9899:1990):
+  * 3.1.9 Comments
+
+### See also
+
+<DescList>
+  <Desc>
+    <DocLink slot="item" src="/cpp/comments">C++ documentation</DocLink> for <span>Comments</span>
+  </Desc>
+</DescList>