doc: include line/cursor in readline documentation

Documents the existence and purpose of the `line` and `cursor`
properties.  `line` can be used for reading the current input value
during runtime, if reading from a TTY stream.  Both properties are
necessary when developing a custom CLI input process using `readline`
as a backend.

Refs: https://github.com/nodejs/node/issues/30347
Refs: https://github.com/DefinitelyTyped/DefinitelyTyped/pull/40513

PR-URL: https://github.com/nodejs/node/pull/30667


Reviewed-By: Anna Henningsen <anna@addaleax.net>