Docs: regex replace in post-processing and document capture groups

[wolfv]

We have the post-processing with regex's. There is a somewhat surprising feature with Rust regex' crate that uses $FOO syntax to expand a named capture group. This should be documented so that others don't fall into this trap :)

E.g. if you want to replace /bla/sys/prefix with $ENV{PREFIX} you need to write $$ENV{PREFIX} otherwise it gets replaced as {PREFIX}.

[wolfv]

I added some docs for this! :)