docs: mention regexps for the interface plugin

docs: mention regexps for the interface plugin

It is very useful to collect data (or ignore) from a group of interfaces
that are similarly named - especially if the list is dynamic and it's
not feasible to update configuration each time an interface shows up or
disappears. It's possible to use regexps to specify interfaces to
collect data from (or ignore), but it's not clearly stated anywhere in
the documentation.
This is an attempt to improve documentation in this area.

Signed-off-by: Jakub Jankowski <shasta@toxcorp.com>