[bazel.build] Problem with /external/extension

[aherrmann]

Page link:

https://bazel.build/external/extension

Problem description (include actual vs expected text, if applicable):

As is explained and motivated in #17564 (comment), module extensions are identified by the .bzl file they are imported from instead of reference equality on the underlying module extension object. This is different behavior from most other concepts in Starlark and something that extension authors should be aware of. It would be helpful to document this fact in the Module extensions documentation.

Where do you see this issue? (include link to specific section of the page, if applicable)

This fact is not documented in https://bazel.build/external/extension#extension_definition as of now.

Any other information you'd like to share?

I will open a PR shortly with a proposed addition to the documentation.