regcomp.c: Clarify some comments
Fix broken qr/ \N* /x

Under /x, the parsing was skipping the character following a \N (when
meaning [^\n]).  In fact the parse pointer was already pointing to the
first non-white-space character, so advancing skipped the character.
That character could be the beginning of a (?#...) comment, all of which
should be skipped.
perlre: Expand and clarify /x and (?# comment)

This pod omitted some of the details about how comments in regexes work.
I had to do some experimentation to find some of the answers.
I believe this clarifies things as well.