mediawiki.storage: Document 'null' return from get()

mediawiki.storage: Document 'null' return from get()

Follows-up 7daab75414d. Forgot to document the full return scope
of HTML Storage#get, which is 'string or null'.

We may want to consider changing the 'false' return to 'null', so
that the concept of "localStorage disabled" is entirely hidden
behind the regular return values (null in case of #get).

Change-Id: I6bed62457d2e64831ec8241e16f7e8c85c6531a4