doc: correct crypto.randomFill() and randomFillSync()

Correct return type of `crypto.randomFillSync()` which is of same type as
passed as `buffer` argument.

Correct samples for `randomFill()` and `randomFillSync()` using a `TypeArray`
or `DataView` as these types don't support `.toString(encoding)`.

PR-URL: https://github.com/nodejs/node/pull/21550


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>