Document existing immutable behavior of ArrayAdapter

Various methods on ArrayAdapter throw exceptions if run on an adapter
created with one of the varargs constructors. This is because those contructors
create an Arrays.ArrayList, which does not allow modification. It is not
worth changing the existing behavior, but it is worth making the behavior
clearer in the docs.

Bug: 63802412  ArrayAdapter crashes when calling clear() directly after constructor
Test: docs-only fix. Verified with unit test that the docs apply correctly to the
newly-documented methods. Built the docs target successfully.

Change-Id: I75ae233879cc7db10853f93d78625a00675594b2