docs: howto/cgi.xml tone normalization and mod_cgi/mod_cgid explanation

docs: howto/cgi.xml tone normalization and mod_cgi/mod_cgid explanation

- Add paragraph explaining why two CGI modules exist: mod_cgid for
  threaded MPMs (event, worker), mod_cgi for non-threaded (prefork)
  and Windows; note they are configuration-compatible
- Link to both module reference pages for implementation details
- Clean up LoadModule note block (remove duplicate "A correctly
  configured directive may look like this")
- Remove "Of course" asides and "In order to" constructions
- "There are two main differences" → direct statement
- Move bug-database advisory into <note> block
- Convert remaining Perl references to Python

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1935513 13f79535-47bb-0310-9956-ffa450edef68