(MAINT) Clarify line length for about topics (#11307)

* (MAINT) Clarify line length for about topics

Prior to this change, the contributing guide indicated that the line
length should be limited to 100 characters. It clarified that this
guideline applies to conceptual articles and cmdlet reference, but
didn't indicate that the line length in `about_` topics should be
limited to 80 characters.

This change clarifies the guidance.

* Update reference/docs-conceptual/community/contributing/general-markdown.md

---------

Co-authored-by: Sean Wheeler <sean.wheeler@microsoft.com>

Author: michaeltlombardi

reference/docs-conceptual/community/contributing/general-markdown.md

@@ -1,6 +1,6 @@
 ---
 description: This article provides specific guidance for using Markdown in our documentation.
-ms.date: 04/23/2024
+ms.date: 08/01/2024
 title: Markdown best practices
 ---
 # Markdown best practices
@@ -50,9 +50,9 @@ Only use [ATX headings][07] (`#` style, as opposed to `=` or `-` style headers).
 
 ## Limit line length to 100 characters
 
-This applies to conceptual articles and cmdlet reference. Limiting the line length improves the
-readability of `git` diffs and history. It also makes it easier for other writers to make
-contributions.
+This applies to conceptual articles and cmdlet reference. For `about_` topics, limit the line
+length to 79 characters. Limiting the line length improves the readability of `git` diffs and
+history. It also makes it easier for other writers to make contributions.
 
 Use the [Reflow Markdown][09] extension in VS Code to reflow paragraphs to fit the prescribed line
 length.