Docs: Env attributes should be modified using env setters

[fracapuano]

This PR aims at solving #1573.
In the issue, I documented how directly accessing environment attributes might have an unexpected outcome when dealing with Vectorized environments, and that one should instead invoke the env_method method of VecEnv passing in a setter for the environment considered.

This is relevant in use-cases where one uses callbacks modifying environments during training (typical in multi-task/meta-RL scenarios).
In this PR, I present:

• The problem, in full detail
• A use case where this is particularly relevant
• An example solution

Description

I have added a new paragraph to vec_envs.rst, explaining in detail the rationale behind this problem. I have also presented an example of use case where this very same problem might arise, and a scheme of the solution (both on the environment and callback side).

Motivation and Context

This change introduce a new paragraph into the documentation aiming at solving the unexpected behavior of directly accessing environment attributes via dot notation.

#1573

closes #1573

• [ x] I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

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• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have opened an associated PR on the SB3-Contrib repository (if necessary)
• I have opened an associated PR on the RL-Zoo3 repository (if necessary)
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[fracapuano]

hey @pseudo-rnd-thoughts @araffin, just following up on this PR (proposing a doc integration as per #1573)

[pseudo-rnd-thoughts]

I know that SB3 uses different vector implementations from the Gymnasium ones so I would just check that this still works if the environment is wrapped by a wrapper, e.g., ClipAction

[fracapuano]

I know that SB3 uses different vector implementations from the Gymnasium ones so I would just check that this still works if the environment is wrapped by a wrapper, e.g., ClipAction

on it

[araffin]

LGTM, thanks =)