Add documentation for user defined literal operator and usage

Author: mbaluda

cpp/common/src/codingstandards/cpp/UserDefinedLiteral.qll

@@ -4,6 +4,15 @@
 
 import cpp
 
+/**
+ * A user defined literal operator is a function that defines the behavior of a user defined literal. 
+ * It is declared using the `operator ""` syntax.
+ * ```
+ * constexpr long operator""_km(unsigned long value) {
+ *   ...
+ * }
+ * ```
+ */
 class UserDefinedLiteralDeclaration extends Function {
   UserDefinedLiteralDeclaration() {
     // We use the '?' in this regexp because CodeQL CLI 2.4.6 and earlier reported these operators
@@ -17,6 +26,12 @@ class UserDefinedLiteralDeclaration extends Function {
   predicate hasCompliantSuffix() { this.getName().regexpMatch("operator \"\"?_\\p{Alpha}+") }
 }
 
+/**
+ * A user defined literal is a literal that is passed as an argument to a call to a user defined literal operator.
+ * ```
+ * 1000_km;
+ * ```
+ */
 class UserDefinedLiteral extends Literal {
   UserDefinedLiteral() {
     exists(FunctionCall fc |