Proto/Scoru: simple fields renaming in history to auto-document.

Context

Use better names for fields in history fields to auto-document the code and avoid ambiguity.

Also, add docstring for all the fields of history datatype.

Another MR will add an additional field (oldest_index) and fix a bug (two) reported here #2981 (closed) and exhibited here !5916 (merged) (for one of them)

Manually testing the MR

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