Add form description for settings fields

Maybe this is a good novice job, if a novice reads into the README or the code and adds the #description using the Form API for this settings form?

Hello @anybody,
I have read the Readme file and the functionality and updated the description of fields of settings form.
Please review.

Thanks @divyansh.gupta - really nice and helpful additions. I left two comments, maybe a maintainer could also have a look for final review afterwards. Thank you! Really nice!

@anybody,
I have made the changes please review.

Thanks @divyansh.gupta! Great work. I think now a maintainer should decide, if all important information is given and if it's all fine!

Let's set this RTBC! :)

Those enhancements look good to me, just merged them. Thank you all for your contributions.