Help: Clarify documentation on regex character classes

CMake regex parsing was giving me troubles when trying to match a character class that contains a closing bracket ].

I could only get string(REPLACE REGEX ...) to work if if put the bracket right after the opening bracket e.g. []ab].

Any number of backslashes (tried up to 5) didn't allow me to put the closing bracket anywhere else. [a]b] doesn't work (as expected). But also [a\]b] or [a\\]b] etc. don't give me the desired result.

Updated the documentation in the same style as it's done for a literal - in character classes.

Topic-rename: doc-regex-classes