nm: document -S flag

R=r
CC=golang-dev
https://golang.org/cl/4247066

src/cmd/nm/doc.go

@@ -11,6 +11,9 @@ Nm is a version of the Plan 9 nm command.  The original is documented at
 It prints the name list (symbol table) for programs compiled by gc as well as the
 Plan 9 C compiler.
 
+This implementation adds the flag -S, which prints each symbol's size 
+in decimal after its address.
+
 For reasons of disambiguation it is installed as 6nm although it also serves
 as an 8nm and a 5nm.