basic_string::replace description : fix missing words.
Merge pull request #253 from faithandbrave/fix_basic_string_replace_typo

[string::replace]/35 Add missing `returns` command

Reported by Akira Takahashi.
[func.bind.isbind]/2 says "have a BaseCharacteristics"

Reported by Stephan T. Lavavej.

Closes #259.
[pointer.traits.functions] Add paragraph numbers.

Reported by Stephan T. Lavavej. Closes #258.
[util.smartptr.shared.const]/33 Add whitespace

Reported by Stephan T. Lavavej. Closes #249.
[pair.astuple] Undo reference to tuple_element_t.

Reported by Michael Park. Closes #247.
[pair.astuple] Give names to function parameters.

Reported by Stephan T. Lavavej. Closes #231.
[re.synopt] Use code font for match_results.