append_revisions_only accept all valid booleans, update doc to warn about backward compatibility.
* bzrlib/tests/test_branch.py: (TestBranchOptions.check_aro_is): Update warning message. (TestBranchOptions.test_valid_append_revisions_only): More values are accepted. (TestBranchOptions.test_invalid_append_revisions_only): Less values are invalid.
* bzrlib/tests/blackbox/test_init.py: (TestSFTPInit.test_init_append_revisions_only): The default value is now None.
* bzrlib/branch.py: (BzrBranch8._get_append_revisions_only): Accept all valid booleans.