Document that second parameter of WP_Dependencies::add() can be boolean

[dimadin]

Although documentation states that second parameter of WP_Dependencies::add() is a string, it can be boolean false, used when item is an alias for one or more other dependencies.

This is done, for example, for jquery, wp-admin, and few other in core. It is expected for both styles and scripts.

[SergeyBiryukov]

In 43661:

Docs: Document that the $src parameter of WP_Dependencies::add() can be boolean.

Props dimadin.
Fixes #45009.