Relational: refactor internal methods, minor comment/docstring fixes

Each concrete class must implement `_eval_relation`.  Docs no longer
say this represents pure real number comparison.  For that, add a new
`_eval_real_number_relation`.  This defaults to `_eval_relation` but
subclasses such as Equality can override.  `.doit` will now call
`_eval_relation` (drop the intermediate `_eval_relation_doit`).