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
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines