refactor title.py - #2657

Minor refactor after conversation with @scanny

Updates docstring and how chunking options are accessed.
`self._kwargs.get()` should only be used in the `lazyproperty`
definition of an instance's attribute. Other calls should use
`self.<attribute>`

Minor refactor after conversation with @scanny

Updates docstring and how chunking options are accessed.
`self._kwargs.get()` should only be used in the `lazyproperty`
definition of an instance's attribute. Other calls should use
`self.<attribute>`

`combine_text_under_n_chars` is documented as capped at `new_after_n_chars`:
a combine-threshold above the soft-max recombines pre-chunks the soft-max
has already declared full, so the caller gets chunks well past the size
they asked for.

The cap was implemented until the `title.py` refactor in 9ac4445 (Unstructured-IO#2657),
which replaced

    return soft_max if combine_text_under_n_chars > soft_max else combine_text_under_n_chars

with a plain read of the argument, while leaving both the property's own
docstring ("Is reduced to `new_after_n_chars` when it exceeds that value")
and the public option documentation in `base.py` ("Note this value is
'capped' at the `new_after_n_chars` value") describing the old behavior.

    chunk_by_title(
        elements, max_characters=500, new_after_n_chars=100,
        combine_text_under_n_chars=400,
    )

returned one 367-character chunk where three 121-character chunks were
asked for.

Restore the cap. Validation now reads the argument as the caller gave it
rather than the capped property, so a negative value and a value greater
than `max_characters` are both still rejected with the same messages; the
cap would otherwise have hidden the second case.