mod_rewrite: Clarify Substitution description

The opening sentence of the Substitution section implied that Pattern
always matches when a rule fires, which is incorrect for negated rules.
Reword to be neutral about how the rule was triggered, and add a
cross-reference to "What is matched?" for context.

Resolves bz#70061


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1935194 13f79535-47bb-0310-9956-ffa450edef68