Clarify MergePropTypes
Clarify MergePropTypes

dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,borisyankov/DefinitelyTyped,borisyankov/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped,dsebastien/DefinitelyTyped