tests: try several pool min_size updates relative to size

Signed-off-by: Dan Mick <dan.mick@inktank.com>
doc: note special handling of min_size in pool create/modify

Signed-off-by: Dan Mick <dan.mick@inktank.com>
doc: missing module level in manhole notes

Signed-off-by: Dan Mick <dan.mick@inktank.com>