I think this information is already given in the sentences before.
Index: mmap.2
===================================================================
RCS file: /cvs/src/lib/libc/sys/mmap.2,v
retrieving revision 1.39
diff -u -p -u -p -r1.39 mmap.2
--- mmap.2 12 Apr 2012 12:53:27 -0000 1.39
+++ mmap.2 6 Oct 2012 19:35:32 -0000
@@ -188,10 +188,6 @@ The symbol
.Dv MAP_FAILED
is defined in the header
.Ao Pa sys/mman.h Ac .
-No successful return from
-.Fn mmap
-will return the value
-.Dv MAP_FAILED .
.Sh ERRORS
.Fn mmap
will fail if:
