Clarify phpdoc return types for template helper functions

[manhphucofficial]

Some template helper functions currently document their return value as mixed,
even though they only return well-defined values.

This ticket proposes clarifying the PHPDoc return types to better reflect the
existing behavior, without changing any runtime logic.

[manhphucofficial]

A documentation-only pull request has been opened to address this:
​https://github.com/buddypress/buddypress/pull/433

This PR updates a couple of PHPDoc @return annotations that currently use
mixed, even though the functions return well-defined values.

The changes clarify the return types based on the existing behavior:

• bp_format_time() returns a formatted string on success or false on failure.
• bp_get_activity_parent_content() returns a string or false when no parent content is available.

This is a documentation-only change and does not affect runtime behavior.

This PR addresses Trac ticket #9319:
​https://buddypress.trac.wordpress.org/ticket/9319