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

Pinging previous reviewers: @gireeshpunathil @vsemozhetbyt @jasnell @trivikr

Checklist

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