-mark is active, and there's a valid region in the buffer. Commands
-that operate on the region (instead of on text near point) when
-there's an active mark should use this to test whether to do that.
+mark is active, and there is a valid region in the buffer. This
+function is intended to be used by commands that operate on the
+region, instead of on text near point, when the mark is active.
+
+@cindex empty region
+@vindex use-empty-active-region
+A region is valid if it has a non-zero size, or if the user option
+@code{use-empty-active-region} is non-@code{nil} (by default, it is
+@code{nil}). The function @code{region-active-p} is similar to
+@code{use-region-p}, but considers all regions as valid. In most
+cases, you should not use @code{region-active-p}, since if the region
+is empty it is often more appropriate to operate on point.