Make regex-matching docs more obvious
Make regex-matching docs more obvious

Split out the note about regex-matching into a separate paragraph to
make it more obvious. Move it up, closer to the definition.

Signed-off-by: SuperQ <9bbf917dc63fbaa40c67921332d26cf16a50db75@gmail.com>

prometheus/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,prometheus/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus