Update old contributing docs to match AUTHORS docs (#20486)

The AUTHORS file itself already mentioned this.

Context: #20480

site/source/docs/contributing/index.rst

@@ -8,7 +8,8 @@ This section contains articles that are relevant to anyone who wants to contribu
 
 - :ref:`contributing` provides an introduction to contributing, along with a few ideas for how you can help.
 - The :ref:`Developer's-Guide` contains contributor-relevant information, covering compiler operation and submitting patches to the Emscripten repositories.
-- :ref:`emscripten-authors` is a list of all Emscripten contributors. If you contribute you must accept the project’s :ref:`open source licenses (MIT/LLVM) <emscripten-license>` and add yourself to the list!
+- :ref:`emscripten-authors` is a partial list of Emscripten contributors, which used to be updated manually. The git log has the full list of all contributors,
+  and for that reason it is not necessary to add to the AUTHORS file manually any more (but it is optional).
 
 .. toctree::
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