Documentation: Clarify a few subtleties

* Clarify accepted values of <LANG> suffix in CMAKE_COMPILER_IS_GNU<LANG>.

* Clarify fact that target property GENERATOR_FILE_NAME usually cannot be
  used at configure time.

* Clarify that enable_language() can only be used in global scope of
  top-level project using language.

* State that enable_language(... OPTIONAL) currently doesn't work.

* Document regular expression operator precedence.
Merge topic 'doc-fixes' into next

3cf2f67 Documentation: Clarify a few subtleties