Clarifies capturing properties of square brackets in regex

[MorayJ]

Also clarifies how round brackets work in name capture.

Also take the mystery out of what we are going to see in subrules.

The problem

named capture using non-capturing group confusion #1634

Also looked like a little further clarity could be added to the way brackets work for capturing.

Solution provided

Refer to 'default' of square brackets not capturing and then highlight that it is a change to default behaviour that they capture when named. Didn't want to use round brackets in the example as think it's advisable myself (though not imperative) to use square brackets in named groupings. Highlighted that you can use round brackets in the example though.

Added a line mentioning named regexes as this is what the subrules link goes on to talk about, but no harm mentioning it here.