cmd/go: clarify @none effect on dependants modules

Expand modules documentation to clarify why @none is useful. The
wording is the one suggested by rsc on the issue.

Fixes #26684

Change-Id: I76dc4ff87e50f1dd8536fd9ac1fd938adb29bee3
Reviewed-on: https://go-review.googlesource.com/c/161037Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/alldocs.go

@@ -2496,7 +2496,8 @@
 // development module, then get will update the required version.
 // Specifying a version earlier than the current required version is valid and
 // downgrades the dependency. The version suffix @none indicates that the
-// dependency should be removed entirely.
+// dependency should be removed entirely, downgrading or removing modules
+// depending on it as needed.
 //
 // Although get defaults to using the latest version of the module containing
 // a named package, it does not use the latest version of that module's

src/cmd/go/internal/modget/get.go

@@ -56,7 +56,8 @@ If a module under consideration is already a dependency of the current
 development module, then get will update the required version.
 Specifying a version earlier than the current required version is valid and
 downgrades the dependency. The version suffix @none indicates that the
-dependency should be removed entirely.
+dependency should be removed entirely, downgrading or removing modules
+depending on it as needed.
 
 Although get defaults to using the latest version of the module containing
 a named package, it does not use the latest version of that module's