Docs: Update inline docs for General template functions

[dilipbheda]

I reviewed the following function and found that the inline docs are missing the null param type and default value description. e.g: Default false.

• get_header()
• get_footer()
• get_sidebar()
• get_template_part()
• get_archives_link()

This looks good to me. Please note that I'll fix a small indentation issue on line 200 when committing this change.

[audrasjb]

In 60120:

Docs: Update inline docs for various general template functions.

Props dilipbheda, mukesh27.
Fixes #63215.
See #63166.

[SergeyBiryukov]

In 60121:

Docs: Correct the type of the $name parameter in get_template_part action.

The value cannot be null at this point, as it is converted to a string a few lines above.

Follow-up to [45059], [55870], [60120].

See #63215, #63166.

[peterwilsoncc]

Reopening and moving to the 6.8 milestone for r60120, r60121 to be considered for merging to the 6.8 branch.

[jorbin]

In 60139:

Docs: Update inline docs for various general template functions.

Reviewed By peterwilsoncc, jorbin.
Merges [60120] and [60121] to the 6.8 branch.

Props dilipbheda, mukesh27, SergeyBiryukov, audrasjb.
Fixes #63215.
See #63166.