Test_round_repr: Add tests and docs for rounds and timestamps calculation

What

Adding docs and a few more explanatory tests for better clarity.

Why

How

Manually testing the MR

dune exec src/proto_alpha/lib_protocol/test/unit/main.exe \
                  -- --file test_round_repr.ml

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR