refactor: enhance clarity by changing 'parse_secret_string_as_json' option to 'json

[isatyamks]

Updated the docstring to reflect the correct option name json instead of the incorrect parse_secret_string_as_json

    If the secret contains a string payload ("SecretString"):
    - if the `json` option is True (default):
        {SecretString} will be parsed as a JSON. If successfully parsed, AND the JSON contains a
        top-level object, each entry K/V in the object will also be converted to an entry in the result. V will
        always be casted to a string (if not already a string).
    - If `json` option is False:
        {SecretString} will be returned as a single entry in the result, with the key being the secret_id.n.

[saikonen]

as we're updating the docstring to match behaviour, could you also add for the json=False case that the returned secret key is either the secret_id OR the value set by options={"env_var_name": custom_env_var_name}. I leave the phrasing up to you :)

see L149:153 for that code path

[isatyamks]

hey @saikonen Now i think the docstring is aligned with the actual behavior and provides a clear explanation of the json=False case.

[isatyamks]

@saikonen Since the PR has already been approved and all checks have passed, could you please proceed with merging it?