apicompat --package-assembly-references is difficult to use

The docs/usage say it should be `TFM=file,file,` but it's actually a `tfm|file,file`.

The docs don't indicate how to specify more than one TFM - but this is done by passing the parameter multiple times.

The docs say `TFM` but it actually needs the full form of the moniker. It should allow the short form (by parsing it).

The docs should also indicate that a directory works.

Alternate option for a package - generate a multi-targeting project for all frameworks discovered in package, add a reference to the package, restore it, and then drive API compat off such a project. This could save a lot of manual inputs, but it would be a lot to juggle (MSBuild reference, temporary project, etc etc).