Add description for -Os and -Oz in rustc.1

Author: tesuji

src/doc/man/rustc.1

@@ -261,7 +261,7 @@ full debug info with variable and type information.
 .RE
 .TP
 \fBopt\-level\fR=\fIVAL\fR
-Optimize with possible levels 0\[en]3
+Optimize with possible levels 0\[en]3, s (optimize for size), or z (for minimal size)
 
 .SH ENVIRONMENT