[Documentation] Add description for VoteTally's methods (#3140)

# Description

While methods' names on [`VoteTally`][1] trait might be self-explanatory
at first sight, the distinction between `support` and `approval` can be
a bit ambiguous for some readers. This PR aims to clarify the
distinction and inform about the expected values for every not yet
documented method on this trait.

# Checklist

- [x] My PR includes a detailed description as outlined in the
"Description" section above
- [ ] My PR follows the [labeling requirements](CONTRIBUTING.md#Process)
of this project (at minimum one label for `T`
  required)
- [x] I have made corresponding changes to the documentation (if
applicable)
- [x] I have added tests that prove my fix is effective or that my
feature works (if applicable)

[1]:
https://docs.rs/frame-support/latest/frame_support/traits/trait.VoteTally.html

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Co-authored-by: Bastian Köcher <[email protected]>