doc: state callback behavior on empty buffer

PR-URL: #22461
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Vse Mozhet Byt <[email protected]>

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.