Add missing docstrings

[sloede]

          @ArseniyKholod Could you please complete the missing docstrings for the `EncodingParams` methods in https://github.com/sloede/OpenFHE.jl/pull/30/files#diff-3a747695ef4a43551f061b9a00dba95335581cb549fcd23e707c42ae388d0440R119-R226? I've hopefully marked all locations that need changes with `FIXME`. Feel free to directly push into this branch, I will not modify it unless I notify you via Slack.

Once this PR is complete, we can create a new release and then set up SecureArithmetic.jl to automatically extract the default batch size from the CryptoContext

Originally posted by @sloede in #30 (comment)