doc: correct filehandle.[read|write|append]File()

* Remove `mode` and `flag` options for all three functions, since they
all ignore those options.
* Remove mention of `path` argument for `readFile`, since it isn't
actually an option to this method.
* Clarify that for file handles, `appendFile` and `writeFile` are
equivalent.

PR-URL: https://github.com/nodejs/node/pull/31235


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>