doc: revert incorrect change on readable._read

https://github.com/nodejs/node/pull/17979 introduced a change in the
doc that was not correct about _read always being called asynchronously.
This does not hold true when it is in flowing mode.

See: https://github.com/nodejs/node/pull/17979
Fixes: https://github.com/nodejs/node/issues/24919

PR-URL: https://github.com/nodejs/node/pull/25442


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>