Fix docstring for log_interval to differentiate between on-policy/off-policy logging frequency

[rushitnshah]

Description

• closes PPO log interval on Stable baselines 3. #1847.
• Updates documentation/docstring for the parameter log_interval in the base class.

Motivation and Context

• The documentation describes the log_interval parameter of PPO's learn method as the "number of episodes before logging".
• However, this description is applicable only to off-policy algorithms. Because of how PPO works logging is actually performed after every log_interval*n_steps steps of rollout collection (not log_interval episodes as the documentation describes.) (See Issue PPO log interval on Stable baselines 3. #1847)
• This description comes from the docstring for the learn method inside base_class.py which I have updated to differentiate how log_interval is employed for on-policy v/s offpolicy.

• 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)

Checklist

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• I have updated the changelog accordingly (required).
• 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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• I have checked that the documentation builds using make doc (required)

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[araffin]

LGTM, thanks =)