Add information on junctions to wire documentation

A while ago, I added junctions to some wires. Today, I needed to do it again, and couldn't remember what keys to hold down while dragging. The documentation doesn't mention junctions at all.

Once I found out that they were called junctions (searching for "node red split wire" doesn't produce useful results) I found something in the 3.0 release notes but it took over 10 minutes in all.

Could something be added to the documentation covering this feature? I can submit a PR for the text, but I'm not really in a position to generate helpful images at the moment unless I just lift the ones from the release notes.

wire のドキュメントページから始め、junctions に関するリンク先の Node-RED 3.0 リリースノートと比較してください。junctions の作成方法（ドラッグ中に押しておくキーを含む）を文書化し、利用可能であれば役立つ画像を追加してください。ドキュメントだけで、ユーザーが他の場所を検索せずにこの機能を見つけて使用できるようにする必要があります。