_content/ref/mod: document dotless module paths

Fixes golang/go#32819
For golang/go#37641

Change-Id: Icbf33337f1b35d47a44ddd7512ce4f7d0f8a0d56
Reviewed-on: https://go-review.googlesource.com/c/website/+/359594
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: Bryan C. Mills

Diff for: _content/ref/mod.md

@@ -72,6 +72,16 @@ so that the `go` command can find and download the module. There are also
 several [lexical restrictions](#go-mod-file-ident) on characters allowed in
 module paths.
 
+A module that will never be fetched as a dependency of any other module may use
+any valid package path for its module path, but must take care not to collide
+with paths that may be used by the module's dependencies or the Go standard
+library. The Go standard library uses package paths that do not contain a dot in
+the first path element, and the `go` command does not attempt to resolve such
+paths from network servers. The paths `example` and `test` are reserved for
+users: they will not be used in the standard library and are suitable for use in
+self-contained modules, such as those defined in tutorials or example code or
+created and manipulated as part of a test.
+
 ### Versions {#versions}
 
 A <dfn>version</dfn> identifies an immutable snapshot of a module, which may be