Add html-, mhtml- and python-mode support to semantic symref

Add html-, mhtml- and python-mode support to semantic symref

* lisp/cedet/semantic/symref/grep.el
(semantic-symref-filepattern-alist): Fix the entry for
'html-mode', which used a regexp-like syntax where only glob
syntax is permitted.  As a result, 'xref-find-references' (M-?)
can now find references in HTML files.  Also duplicate the same
entry for the sake of 'mhtml-mode', and add a new one for
'python-mode'.
(semantic-symref-derive-find-filepatterns): In the documentation,
clarify that returned patterns must follow the glob syntax.  Fix
an 'if' test that always evaluates to nil.
(semantic-symref-tool-grep):
(semantic-symref-perform-search): Fix typos.