cmd/go: fix documentation for exported functions

I accidentally submitted https://golang.org/cl/13080 too early.

Update #11955.

Change-Id: I1a5a6860bb46bc4bc6fd278f8a867d2dd9e411e1
Reviewed-on: https://go-review.googlesource.com/13096Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/go/alldocs.go

@@ -781,14 +781,14 @@ are:
 	-buildmode=c-archive
 		Build the listed main package, plus all packages it imports,
 		into a C archive file. The only callable symbols will be those
-		functions marked as exported by the cgo tool. Requires exactly
-		one main package to be listed.
+		functions exported using a cgo //export comment. Requires
+		exactly one main package to be listed.
 
 	-buildmode=c-shared
 		Build the listed main packages, plus all packages that they
 		import, into C shared libraries. The only callable symbols will
-		be those functions marked as exported by the cgo tool. Non-main
-		packages are ignored.
+		be those functions exported using a cgo //export comment.
+		Non-main packages are ignored.
 
 	-buildmode=default
 		Listed main packages are built into executables and listed

src/cmd/go/help.go

@@ -548,14 +548,14 @@ are:
 	-buildmode=c-archive
 		Build the listed main package, plus all packages it imports,
 		into a C archive file. The only callable symbols will be those
-		functions marked as exported by the cgo tool. Requires exactly
-		one main package to be listed.
+		functions exported using a cgo //export comment. Requires
+		exactly one main package to be listed.
 
 	-buildmode=c-shared
 		Build the listed main packages, plus all packages that they
 		import, into C shared libraries. The only callable symbols will
-		be those functions marked as exported by the cgo tool. Non-main
-		packages are ignored.
+		be those functions exported using a cgo //export comment.
+		Non-main packages are ignored.
 
 	-buildmode=default
 		Listed main packages are built into executables and listed