C#: Add missing QlDoc for various predicates

Author: tamasvajk

csharp/ql/src/semmle/code/cil/Access.qll

@@ -31,6 +31,7 @@ class ReadRefAccess extends ReadAccess, ReadRef { }
 
 /** An instruction that writes a variable. */
 class WriteAccess extends VariableAccess, @cil_write_access {
+  /** Gets the `Expr` whose value is used in this variable write. */
   Expr getExpr() { none() }
 }
 

csharp/ql/src/semmle/code/csharp/Callable.qll

@@ -437,6 +437,9 @@ class Operator extends Callable, Member, Attributable, @operator {
 
   override string getName() { operators(this, _, result, _, _, _) }
 
+  /**
+   * Gets the metadata name of the operator, such as `op_implicit` or `op_RightShift`.
+   */
   string getFunctionName() { none() }
 
   override ValueOrRefType getDeclaringType() { operators(this, _, _, result, _, _) }

csharp/ql/src/semmle/code/csharp/Conversion.qll

@@ -514,6 +514,11 @@ predicate convNullableType(ValueOrRefType fromType, NullableType toType) {
   )
 }
 
+/**
+ * Holds if `fromType` is `NullType`, and `toType` is a type that can represent
+ * the `null` value, such as a reference type, `Nullable<T>` or a type parameter
+ * with contraints that restrict it to a reference type.
+ */
 // This is a deliberate, small Cartesian product, so we have manually lifted it to force the
 // evaluator to evaluate it in its entirety, rather than trying to optimize it in context.
 pragma[noinline]

csharp/ql/src/semmle/code/csharp/dispatch/RuntimeCallable.qll

@@ -26,6 +26,7 @@ class RuntimeMethod extends RuntimeCallable {
     this instanceof CIL::Method
   }
 
+  /** Holds if the method is `static`. */
   predicate isStatic() { this.(Method).isStatic() or this.(CIL::Method).isStatic() }
 }
 

csharp/ql/src/semmle/code/csharp/exprs/Expr.qll

@@ -1011,6 +1011,7 @@ class RangeExpr extends Expr, @range_expr {
 
 /** An index expression, for example `^1` meaning "1 from the end". */
 class IndexExpr extends Expr, @index_expr {
+  /** Gets the sub expression of this `IndexExpr`. */
   Expr getExpr() { result.getParent() = this }
 
   override string toString() { result = "^..." }

csharp/ql/src/semmle/code/csharp/metrics/Coupling.qll

@@ -222,6 +222,10 @@ predicate shareFieldOrProperty(ValueOrRefType t, Method m1, Method m2) {
   )
 }
 
+/**
+ * Holds if `Method` `m` is declared in `ValueOrRefType` `t`, and if there's a
+ * `FieldAccess` or `PropertyAccess` in `m` that accesses `Declaration` `d`.
+ */
 predicate methodUsesFieldOrProperty(ValueOrRefType t, Method m, Declaration d) {
   m.getDeclaringType() = t and
   (