Docs: Correct lone `@param null` usages around some `$deprecated` vars.

Update incorrect uses of `null` as a single parameter type to be `mixed` instead, largely targeting `$deprecated` variable names.

Pipe `|` a few `null`s with other valid types where appropriate.

Add a missing empty new-line between the `_deprecated_file()` and `_deprecated_argument()` functions.

See #64224.

git-svn-id: https://develop.svn.wordpress.org/trunk@61995 602fd350-edb4-49c9-b593-d223f7449a82