There seems to be a "be" missing in install(1).
The whole phrase seems odd to a non-native speaker like me:
those options "should not be relied upon" because they are not portable,
not "for portability". (But I might easily be wrong.)
Jan
--- install.1.orig Mon Feb 1 13:57:31 2016
+++ install.1 Mon Feb 1 13:58:00 2016
@@ -215,7 +215,7 @@ The
.Fl p ,
and
.Fl S
-flags are non-standard and should not relied upon for portability.
+flags are non-standard and should not be relied upon for portability.
.Pp
Temporary files may be left in the target directory if
.Nm
