malloc.3: Clarify that realloc() may move the memory block

Make it clearer, early in the discussion, that realloc()
may move the block.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/malloc.3

@@ -163,7 +163,8 @@ is done.
 .SS reallocarray()
 The
 .BR reallocarray ()
-function changes the size of the memory block pointed to by
+function changes the size of (and possibly moves)
+the memory block pointed to by
 .I ptr
 to be large enough for an array of
 .I nmemb