Document callin LayoutButtons

It's a very important callin, many widget managers use it for hooking `SelectionChanged` `CommandsChanged` and I still don't have any idea of the impact of the return values.

One of the many gotchas within this callin is that `Spring.GetActiveCmdDescs` for example does not work.

Locate the LayoutButtons callin and related widget-manager usage, then trace how SelectionChanged and CommandsChanged are hooked and how return values affect behavior. Verify the documented behavior against Spring.GetActiveCmdDescs, including the reported limitation, and finish when the impact and known gotchas are clearly recorded.

Written by the indexing model from the issue text.