Expand RichText/RichBlock subclass docstrings to match project format

Addresses review feedback on PR #2600: the 25 RichText* and 24 RichBlock*
subclasses used condensed one-line docstrings, unlike the rest of the
library. Rewrite them in the project's reStructuredText format with a
description, Telegram documentation link, per-parameter :param:/:type:,
and :return:/:rtype:, matching the base classes. Docs-only; no logic
changes.

Author: ranjitsingha