t/re/charset.t: Better test descriptions
pod/perlrecharclass: Nits
pod/perlrecharclass: Document [:blank:], [:ascii:] for C89 platforms

These POSIX classes were introduced in C99, and not all C89 platforms
recognize them.  The behavior there was previously undocumented
regcomp.c: Add some comments
perlfunc: 'if' is in perlsyn, not perlop

Thanks to Hauke D for pointing this out.