regex.3: Destandardeseify Match offsets

regex.3: Destandardeseify Match offsets

This section reads like it were (and pretty much is) lifted from POSIX.
That's hard to read, because POSIX is horrendously verbose, as usual.

Instead, synopsise it into something less formal but more reasonable,
and describe the resulting range with a range instead of a paragraph.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Alejandro Colomar <alx@kernel.org>