Log if channels expand to a smaller set than requested

In a reasonable number of cases, we see issues with access that aren't immediately apparent at INFO level logging and fall into the standard "Sync doesn't work" or "This document doesn't sync" where you request an explicit list of channels and have access to only a subset of them.

You can pick this out by looking at caching logging, but it's not super obvious, and even the best line for this ( "MultiChangesFeed: channels expand to %#v ... %s" ) takes a bit of cross referencing to pick out the issue.

I'd be nice if we could log at the point of filtering if we've dropped some channels from the list to make this more obvious and also potentially aid in self-help on issues like this.

I'd propose WARN as the level, but happy to discuss that further.