This is the second in a series of clean-ups of the

This is the second in a series of clean-ups of the
environment variable docs.  Here is what I have done:

1. Removed some obsolete CERN references and done a tiny
bit of cleanup of mod_env.html.

2. Included references from mod_env.html and mod_setenvif.html
to env.html.

3. Prettied-up env.html a little bit by including sub-headings.

4. Added some "caveats" to env.html:
- No overriding basic CGI variables
- suexec restrictions
- no non-alphanumeric characters in env variable names

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87621 13f79535-47bb-0310-9956-ffa450edef68