Some updates to the RewriteRule docs, mostly focusing on clearly

describing what can be in the substitution string. Included is
a description of how mod_rewrite decides whether to treat the
substitution as a file-system or URL-path.

PR: 22529


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@561840 13f79535-47bb-0310-9956-ffa450edef68