Docs: Incorrect return type for get_comments_number()

[ocean90]

get_comments_number() doesn't return an integer. It's a numeric string, for compatibility reasons.

[kiranpotphode]

Return type changed from int to string.

[pkostadinov]

Modify return results, add description and unit test

[pkostadinov]

I have modified:

• the function description, to reflect correctly the return type of the function.
• the hard-coded return value, to reflect correctly the function description.
• added new unit test accounting for return type.

[DrewAPicture]

Assigning ownership to mark the good-first-bug as "claimed".

[DrewAPicture]

In 41285:

Docs: Clarify that get_comments_number() can return either a numeric string (for compatibility reasons) or 0 if the post doesn't exist.

Also update the hook doc for the get_comments_number filter to reflect the mixed type for $count.

Props kiranpotphode for the initial patch.
Fixes #38369.