doc: fix up warning text about character devices

The text contained a number of inaccuracies:

- The main thread is never blocked by a stream close.
- There is no such thing as an EOF character on the OS level,
  the libuv level, or the Node.js stream level.
- These streams *do* respond to `.close()`, but pending
  reads can delay this indefinitely.
- Pushing a random character into the stream works only when
  the source data can be controlled; Using the JS `.push()`
  method is something different, and does not “unblock” any threads.

Refs: https://github.com/nodejs/node/pull/21212

PR-URL: https://github.com/nodejs/node/pull/22569


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>