doc: note that zlib.flush acts after pending writes

Describe that `zlib.flush()` may wait for pending writes and
until output is being read from the stream.

Fixes: #3782
PR-URL: #6172
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Robert Jefe Lindstädt <robert.lindstaedt@gmail.com>

Author: addaleax

doc/api/zlib.markdown

@@ -308,6 +308,11 @@ class of the compressor/decompressor classes.
 Flush pending data. Don't call this frivolously, premature flushes negatively
 impact the effectiveness of the compression algorithm.
 
+Calling this only flushes data from the internal zlib state, and does not
+perform flushing of any kind on the streams level. Rather, it behaves like a
+normal call to `.write()`, i.e. it will be queued up behind other pending
+writes and will only produce output when data is being read from the stream.
+
 ### zlib.params(level, strategy, callback)
 
 Dynamically update the compression level and compression strategy.