Documentation for `get_header` and `get_footer` actions are not accurate.

[desrosj]

The get_header and get_footer action hook Docblocks both say "The hook allows a specific footer/header template file to be used in place of the default footer template file."

This is not entirely accurate. While the hooks allow you to do something before the behavior of get_header() or get_footer(), it does not allow you to prevent the default output of these functions as the description suggests.

[SergeyBiryukov]

I think we can just remove these inaccurate parts. We generally don't provide this kind of extended explanations for other filters, like get_template_part_{$slug} or pre_get_search_form.

get_sidebar() is also affected.

[SergeyBiryukov]

In 40955:

Docs: Remove inaccurate parts of get_header, get_footer, and get_sidebar hooks documentation.

These sentences suggested that the hooks allow a specific template file to be used in place of the default file, which is not the case.

Props desrosj.
Fixes #40892.