cmd/go: update helpdoc.go about '-buildmode'

After https://golang.org/cl/46421 is landed.
helpdoc.go should be updated that -buildmode=c-shared
requires only one main package.

Fixes #15082

Change-Id: I30a0ee956e6c4655c975ecdaa905887bd474952c
Reviewed-on: https://go-review.googlesource.com/46810Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/alldocs.go

@@ -849,10 +849,10 @@
 // 		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
+// 		Build the listed main package, plus all packages it imports,
+// 		into a C shared library. The only callable symbols will
 // 		be those functions exported using a cgo //export comment.
-// 		Non-main packages are ignored.
+// 		Requires exactly one main package to be listed.
 //
 // 	-buildmode=default
 // 		Listed main packages are built into executables and listed

src/cmd/go/internal/help/helpdoc.go

@@ -589,10 +589,10 @@ are:
 		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
+		Build the listed main package, plus all packages it imports,
+		into a C shared library. The only callable symbols will
 		be those functions exported using a cgo //export comment.
-		Non-main packages are ignored.
+		Requires exactly one main package to be listed.
 
 	-buildmode=default
 		Listed main packages are built into executables and listed