doc: explain _writev() API

The exact context of invocation of _writev API is not well known.
Also, the choice between _write and _writev is not well known.
Add a description to make it explicit.

Fixes: #28408
Refs: #28690

Co-authored-by: Parker Bjur <bjur.parker45@gmail.com>

PR-URL: #31356
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/stream.md

@@ -1886,8 +1886,8 @@ methods only.
 
 The `writable._writev()` method may be implemented in addition or alternatively
 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 and if there is buffered data
+from previous writes, `_writev()` will be called instead of `_write()`.
 
 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