Clarify Document and Window's browsing contexts

Previously, a Document could sometimes "have no browsing context". Now,
"a Document's browsing context" is either a browsing context or null.

Previously, "a Window's browsing context" was not defined, despite being
referenced extensively. Now, it is defined to be the Window's associated
Document's browsing context.

Related to #3846, #4363, and #4366.