mrs.3: weak symbol only works for static programs

In dynamically linked programs, malloc_revoke isn't sufficently
preempted when the init() constructor is run so does not work.
Document this to allow the rest of the patch to land.

lib/libc/stdlib/malloc/mrs/mrs.3

@@ -123,7 +123,8 @@ administratively configured preferences
 is set)
 or system-wide defaults.
 .It
-The value of the
+.Sy For stically linked programs,
+the value of the
 .Va malloc_revocation
 variable.
 .Va malloc_revocation
@@ -144,6 +145,8 @@ or
 then
 .Dv AT_BSDFLAGS
 is ignored.
+.Pp
+.Sy This does not work for dynamically linked programs.
 .El
 .Pp
 .\" XXX: this is not tied to mrs and probably belongs somewhere else