Document module extension identity

Closes bazelbuild#17633

Adds a section to the module extension documentation about the identity of module extensions as described in bazelbuild#17564 (comment).

Closes bazelbuild#17634.

PiperOrigin-RevId: 513206202
Change-Id: I8cdc8ef836b0a119911b04aa7efb4f9882c72f54

site/en/external/extension.md

@@ -113,6 +113,25 @@ def _maven_impl(ctx):
   _generate_hub_repo(name = "maven", repo_attrs)
 ```
 
+### Extension identity
+
+Module extensions are identified by the name and the `.bzl` file that appears
+in the call to `use_extension`. In the following example, the extension `maven`
+is identified by the `.bzl` file `@rules_jvm_external//:extension.bzl` and the
+name `maven`:
+
+```python
+maven = use_extension("@rules_jvm_external//:extensions.bzl", "maven")
+```
+
+Re-exporting an extension from a different `.bzl` file gives it a new identity
+and if both versions of the extension are used in the transitive module graph,
+then they will be evaluated separately and will only see the tags associated
+with that particular identity.
+
+As an extension author you should make sure that users will only use your
+module extension from one single `.bzl` file.
+
 ## Repository names and visibility
 
 Repos generated by extensions have canonical names in the form of `{{ "<var>"
@@ -141,4 +160,4 @@ several repo visibility rules:
     *   This might result in a conflict. If the module repo can see a repo with
         the apparent name `foo`, and the extension generates a repo with the
         specified name `foo`, then for all repos generated by that extension
-        `foo` refers to the former.
+        `foo` refers to the former.