This is used by commands that act specially on the region under
Transient Mark mode.
-The return value is t provided Transient Mark mode is enabled and
-the mark is active; and, when `use-empty-active-region' is
-non-nil, provided the region is empty. Otherwise, the return
-value is nil.
+The return value is t if Transient Mark mode is enabled and the
+mark is active; furthermore, if `use-empty-active-region' is nil,
+the region must not be empty. Otherwise, the return value is nil.
For some commands, it may be appropriate to ignore the value of
`use-empty-active-region'; in that case, use `region-active-p'."