posix_memalign.3: Clarify wording on "return value" when size==0

Reported-by: Will Newton <will.newton@linaro.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/posix_memalign.3

@@ -95,8 +95,9 @@ which must be a power of two and a multiple of
 If
 .I size
 is 0, then
-.BR posix_memalign ()
-returns either NULL,
+the value placed in
+.IR "*memptr"
+is either NULL,
 .\" glibc does this:
 or a unique pointer value that can later be successfully passed to
 .BR free (3).