docs: fix description of _writev()

The documentation gave the impression that when the _writev method is
used, it is used instead of _write. However, when _writev is used,
_write loads the first chunk of data, then _writev loads all the
remaining bufferred chunks in the write queue. The docs have been
changed to reflect this behavior.

Fixes: nodejs#28408

doc/api/stream.md

@@ -1754,8 +1754,9 @@ methods only.
 
 The `writable._writev()` method may be implemented in addition to
 `writable._write()` in stream implementations that are capable of processing
-multiple chunks of data at once. If implemented, the method will be called with
-all chunks of data currently buffered in the write queue.
+multiple chunks of data at once. If implemented, the first chunk will be written with 
+`_write()`. When the initial write is complete, buffered chunks in the write queue, if available, 
+will be written all at once with `_writev()`.
 
 The `writable._writev()` method is prefixed with an underscore because it is
 internal to the class that defines it, and should never be called directly by