allocator: require optimal allocations

Add a clarification the documentation that all buffer allocations
should be optimized for the usage bits provided in the descriptor.
This was always assumed to be a requirement of buffer allocations
but it was not explicitly documented.

Bug: 120493579
Test: N/A
Change-Id: Id9e3e1df861d0f32c995f18cabe37d62e7e5ce5c