cmd/link: explain how to find the name to use for -X

Doc change only.

Fixes #18246.

Change-Id: I2c7b181bcec81e124d30702f6fac5aa43bb57c04
Reviewed-on: https://go-review.googlesource.com/34230Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/link/doc.go

@@ -41,6 +41,8 @@ Flags:
 		Set the value of the string variable in importpath named name to value.
 		Note that before Go 1.5 this option took two separate arguments.
 		Now it takes one argument split on the first = sign.
+		To figure out the name to use, run `go tool nm PROGRAM`;
+		usable variables will be tagged "D" or "d".
 	-buildmode mode
 		Set build mode (default exe).
 	-cpuprofile file