docs: clarify `Pool::size` and `Pool::num_idle` (#4398)

`num_idle` was documented as returning connections "active and idle",
which reads as active plus idle. It only counts idle ones.

`size` was documented as "currently active. This includes idle connections" which is incomplete: `try_increment_size()` bumps the counter before the connection is established, so connections still
being opened are counted too.