Include some warnings about vendor_available for fsmgr.

The interface to fsmgr is subpar and while we haven't had time to
revisit it, we absolutely do not want to expose this to vendors in a
way that would force us to keep this interface in the future.

Test: n/a
Change-Id: I970048aa2e45d7f7eca357d052141e90d6e01123
1 file changed