[RLlib] Update docstrings in recurrent encoders.

[simonsays1980]

Why are these changes needed?

Our docstrings mention that a linear output layer follows the last recurrent one. This is incorrect as we do not have a linear output layer.

This PR removes these statements from all docstrings. Instead of a linear output layer each encoder is usually followed in our default implementations by a linear layer in the corresponding heads.

Related issue number

Closes #47625. Related to #47626.

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
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• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
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  • This PR is not tested :(