[Asterisk-doc] extensions.conf section

[Philipp von Klitzing]

Hi Steven,

just noticed we both forgot something:

> > 				> Exact matches can be typed as is, the letter 'X' matches all
> > 				> digits, and the letter 'N' matches 2-9. This is especially
> > 				> useful when defining outside phone number availability. 
> > 
> > Add a remark on . as well
> 
> Okay.

Add "[13-8]" syntax as well. 
And explain why there is match M for 1-9, which I would love to have... 
for me the N is pretty useless ;->

Cheers, Philipp