Docs: Add more explicit description about the `sendreplies` methods

[neo]

Feature Summary and Justification

This pull request provides a more explicit description for the sendreplies methods, which can only be applied to a thing created by the user.

References

• https://www.reddit.com/dev/api/#POST_api_sendreplies

[LilSpazJoekp]

Would you like to add yourself to the Documentation Contributors section in the AUTHORS.rst file?

[neo]

nah it's so minor, thank you for asking though!

[LilSpazJoekp]

Sounds good, thank you!