[Doc] Correcting documentation with respect to postprocess_trajectory

[renos]

Why are these changes needed?

postprocess_trajectory is referred to incorrectly in the rllib-environments documentation. When defining a custom policy, a user never directly modifies Policy.postprocess_trajectory, they define postprocess_fn, which is in turn called by postprocess_trajectory.

Also, in rllib-algorithms I renamed postprocess_trajectory() to Exploration.postprocess_trajectory() to emphasize that this method is part of the Exploration class and not the Policy method, which should not be modified.

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• [ yes] I've included any doc changes needed for https://docs.ray.io/en/master/.
• 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
  • Unit tests
  • Release tests
  • This PR is not tested :(