add an explanation to the sqla requirement

people keep trying to "fix" this to a broken statement, and I now
realize the reasons for this being added are basically lore lost
in the mists of time. So document it.

The wrong fix is no longer blocked in our gate because we force
install newer pip on our devstack nodes (otherwise lots of other
things break).

Change-Id: I048e33d101a0c09adabac6d17cbc0ff23fba19e7
Merge "add an explanation to the sqla requirement"