docs: Normalize "server-variables" to "server variables" in mod_rewrite

docs: Normalize "server-variables" to "server variables" in mod_rewrite

Remove hyphenation from "Server-Variables"/"server-variables" in
mod/mod_rewrite.xml and rewrite/flags.xml — no RFC precedent exists
for the hyphenated form (RFC 3875 uses "meta-variables"). Rework the
module summary to clarify that HTTP headers and timestamps are a
subset of server variables, not separate categories. Link the term
to its definition in RewriteCond.

Addresses: https://bz.apache.org/bugzilla/show_bug.cgi?id=70059

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1934606 13f79535-47bb-0310-9956-ffa450edef68