Add more comments and ASSERTions in the cache allocator

Add more comments and ASSERTions in the cache allocator

It was clear from the thread at [1] that there is some confusion (including
my own) over the semantics of the slab list. Add some more comments and
ASSERTions to better explain.

Also turn a silly secondary 'if' into the 'else' clause it should have
always been.

[1] https://lists.xenproject.org/archives/html/win-pv-devel/2023-06/msg00002.html

Signed-off-by: Paul Durrant <pdurrant@amazon.com>