cmd/ld: document that -X overwrites initialized variables

Fixes #7626.

LGTM=iant
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/144870045

src/cmd/ld/doc.go

@@ -63,9 +63,9 @@ Options new in this version:
 	-w
 		Omit the DWARF symbol table.
 	-X symbol value
-		Set the value of an otherwise uninitialized string variable.
-		The symbol name should be of the form importpath.name,
-		as displayed in the symbol table printed by "go tool nm".
+		Set the value of a string variable. The symbol name
+		should be of the form importpath.name, as displayed
+		in the symbol table printed by "go tool nm".
 	-race
 		Link with race detection libraries.
 	-B value

test/linkx.go

-// $G $D/$F.go && $L -X main.tbd hello $F.$A && ./$A.out
+// $G $D/$F.go && $L -X main.tbd hello -X main.overwrite trumped $F.$A && ./$A.out
 
 // NOTE: This test is not run by 'run.go' and so not run by all.bash.
 // To run this test you must use the ./run shell script.
@@ -12,9 +12,13 @@
 package main
 
 var tbd string
+var overwrite string = "dibs"
 
 func main() {
 	if tbd != "hello" {
-		println("BUG: test/linkx", len(tbd), tbd)
+		println("BUG: test/linkx tbd", len(tbd), tbd)
+	}
+	if overwrite != "trumped" {
+		println("BUG: test/linkx overwrite", len(overwrite), overwrite)
 	}
 }