doc: clarify descriptions of _writev chunks argument

Fixes: #36789
Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: #36822
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>

doc/api/stream.md

@@ -2162,8 +2162,14 @@ user programs.
 
 #### `writable._writev(chunks, callback)`
 
-* `chunks` {Object[]} The chunks to be written. Each chunk has following
-  format: `{ chunk: ..., encoding: ... }`.
+* `chunks` {Object[]} The data to be written. The value is an array of {Object}
+  that each represent a discreet chunk of data to write. The properties of
+  these objects are:
+  * `chunk` {Buffer|string} A buffer instance or string containing the data to
+    be written. The `chunk` will be a string if the `Writable` was created with
+    the `decodeStrings` option set to `false` and a string was passed to `write()`.
+  * `encoding` {string} The character encoding of the `chunk`. If `chunk` is
+    a `Buffer`, the `encoding` will be `'buffer`.
 * `callback` {Function} A callback function (optionally with an error
   argument) to be invoked when processing is complete for the supplied chunks.