match_results docs incorrectly refer to returning strings

[Jonathan Wakely <jwakely.boost@…>]

The documentation for match_results::length and match_results::position both say:

The overloads that accept a string refer to a named sub-expression n. In the event that there is no such named sub-expression then returns an empty string.

Those functions return difference_type not a string

[John Maddock]

(In [61976]) Update regex_match requirements. Fixes #4215.