Improvements to the regex pages

[David Flynn]

In 'Match Results with Subexpressions' it would be useful to say that rm_eo is 
the offset of the character after the end of the substring.

So for example if the string "hello" was matched in "!!hello world!!" rm_so 
would be 2 and rm_eo would be 7.

It would also be useful to state that any unused regmatch_t entries are filled 
with rm_so and rm_eo values of -1.

David