doc: demonstrate a separately-compiled regex set
doc: demonstrate a separately-compiled regex set

This adds an example of an intended pattern where the regex
set is used to find a matching regex, and then a separately
compiled regex is used to find where it matched.

PR #861 
rust-lang-nursery/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang/regex