bpo-33781: audioop: enhance rounding double as int (GH-7447) (GH-7452)

Move the floor() call into fbound() to call floor() on a double
rather than an int. The change should enhance the rounding.

Document also (int)double rounding mode.

(cherry picked from commit 45e4efba7fa2abe61d25e4f8b5bf482e19ff1280)
1 file changed