cmd/link: document limitation of -X

Fixes #26042

Change-Id: Ica16f14a65c03659a19926852cca5e554c99baf1
Reviewed-on: https://go-review.googlesource.com/120935
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/doc.go

@@ -39,8 +39,10 @@ Flags:
 		Print linker version and exit.
 	-X importpath.name=value
 		Set the value of the string variable in importpath named name to value.
+		This is only effective if the variable is declared in the source code either uninitialized
+		or initialized to a constant string expression. -X will not work if the initializer makes
+		a function call or refers to other variables.
 		Note that before Go 1.5 this option took two separate arguments.
-		Now it takes one argument split on the first = sign.
 	-buildmode mode
 		Set build mode (default exe).
 	-cpuprofile file

src/cmd/link/internal/ld/data.go

@@ -828,6 +828,7 @@ func addstrdata1(ctxt *Link, arg string) {
 	strdata[name] = value
 }
 
+// addstrdata sets the initial value of the string variable name to value.
 func addstrdata(ctxt *Link, name, value string) {
 	s := ctxt.Syms.ROLookup(name, 0)
 	if s == nil || s.Gotype == nil {