Clarifies that emitter.listener() returns a copy, not a reference
Resolves issue nodejs#9022

per: nodejs#14596

1. document that a runtime error will occur if you attempt
   to unshift after the end event
2. document that calling read after the end event will return
   null and will not trigger a runtime error

Minor clarifications around Readable._read and Readable.push
to make their implementation/usage easier to understand.

nodejs#14124 (comment)

Fix for nodejs#25559 (Typo in example of util.deprecate() documentation)

Per nodejs#14604,

Document that performing an `unshift` during a read
can have unexpected results. Following the `unshift`
with a `push('')` resets the reading state appropriately.
Also indicate that doing an `unshift` during a read
is not optimal and should be avoided.

per nodejs#14597

Indicate that `'readable'` indicates only that data can
be read from the stream, not that there is actually data
to be consumed. `readable.read([size])` can still return
null. Includes an example that illustrates the point.

Per nodejs#25635 (comment)

Additional refinement to the clarification on the `readable` event