Document the PageResult attributes in a Returns section

[arrufat]

Closes #6, the last part of item 1.

#14 annotated the eight tools that declare an outputSchema as -> PageResult, but the docstrings still did not say what a PageResult holds, and the issue started from the Returns column the retired reference page had. generate_methods.py now ends those docstrings with a Google-style section built from the output schema:

Returns:
    PageResult: The sentence above, with these attributes (None when absent):

    - `url`: URL of the page the call left loaded.
    - `http_status`: Response status of that page's own document. Absent when no response has arrived. 4xx/5xx means an error page, not the content.
    - `title`: Document title. Empty or absent when the page has none.

The descriptions are the browser's, so they change with the schema. The header line says "None when absent" because the schema describes the wire, where a missing field is left out, while PageResult sets the attribute to None.

Each property is checked against PageResult's annotations. If the browser adds an output field the class doesn't expose, generation fails, so the docstring can't name an attribute that isn't there. The CI drift check runs the generator against nightly, so that failure surfaces on the next PR.

Rendered both ways: pdoc puts it under a Returns heading as a list, and the docs site's generate-python-reference.py (run against this branch) emits the same list under **Returns:** for all eight Session methods.

The data tools stay -> Any with no Returns section. MCP gives no shape for them short of an outputSchema, and browser#3694 deliberately left that off, so a typed return for them would need either that browser change or a hand-written map here.