Incomplete `get_the_modified_(date|time)` filter docs

[dlh]

Both get_the_modified_date() and get_the_modified_time() set the $post variable to the return value of get_post(). As noted in code comments within each function, For backward compatibility, failures go through the filter below (see [37866]). However, the "official" filter docs don't specify that null is a possible type for $post. The attached patch would amend the filter docs to include that note.

[dlh]

40845.2.diff​ also amends docs for $the_time to note the possible bool value.

[DrewAPicture]

@dlh Thanks buddy, looks good :-)

[DrewAPicture]

In 40946:

Docs: Clarify that the $post parameters for the get_the_modified_date and get_the_modified_time filters can also be of type null.

Props dlh.
Fixes #40845.