docs: functions returning should start description with "Returns..."

E.g. https://www.boost.org/doc/libs/master/libs/mysql/doc/html/mysql/ref/boost__mysql__date/get_time_point.html

"Return *this as a time_point , without validity checking"

Start with the documentation page linked in the issue and trace it to the corresponding API documentation source. Review descriptions for functions that return values, update them to begin with “Returns...”, and verify the generated documentation uses the corrected wording.

Written by the indexing model from the issue text.