Clarify the behavior of BIO_gets() a bit more

The API contract is more tight than was previously documented -- the returned
string must be NUL-terminated, and the supplied buffer includes space for
the trailing NUL, so the maximum length that can be read in is reduced.

Clarify that the NUL is not included in the returned length, and fix the
spelling of "NUL-terminated" in a nearby spot.

Adjust punctuation to make a modest improvement to the grammar.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>