Different return types for registerTask and registerMultiTask: should be the same?

`registerTask` returns `grunt.task`, presumably to enable chaining. `registerMultiTask`, on the other hand, returns `undefined`. Is this intended behavior? It seems to me like they both should return `grunt.task`.

I ask because I'm updating grunt's TypeScript type definition and would like to have the correct type signature for both of those functions.

Compare the implementations of registerTask and registerMultiTask, then inspect the Grunt TypeScript type definition mentioned in the issue. Determine whether their return behavior is intended and document the matching signature only after confirming the runtime behavior; done means the API behavior and type definition agree.

Written by the indexing model from the issue text.