CheckSymbolExists: Add examples for check_symbol_exists
Add examples in CheckCXXSymbolExists
too. Also clarify the latter's role w.r.t C++ templates.
Topic-rename: CheckSymbolExists-doc
Edited by Brad King
Add examples in CheckCXXSymbolExists
too. Also clarify the latter's role w.r.t C++ templates.
Topic-rename: CheckSymbolExists-doc