Better document the Tab|Sync command

[dwvisser]

I had to stumble my way through #395 to even realize that this is possible in Zellij. The issue text even steers me to Pane commands rather than the correct Tab commands.

I would like this feature to be documented properly on zellij.dev, so others are aware of it.

[imsnif]

This has been implemented (thanks @dwvisser !) and is now even part of the built-in random tips to be discovered at runtime.