doc: state callback behavior on empty buffer

PR-URL: #22461
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>

Author: RubenVerborgh

doc/api/http.md

@@ -738,7 +738,7 @@ The `encoding` argument is optional and only applies when `chunk` is a string.
 Defaults to `'utf8'`.
 
 The `callback` argument is optional and will be called when this chunk of data
-is flushed.
+is flushed, but only if the chunk is non-empty.
 
 Returns `true` if the entire data was flushed successfully to the kernel
 buffer. Returns `false` if all or part of the data was queued in user memory.