source_files key documentation is incomplete

[kornelski]

https://guides.cocoapods.org/syntax/podspec.html#source_files

Says only:

The source files of the Pod.

• What is meant by a source file? Is it only *.h and *.m? Does it include plists? Does it include language bundles? Does it include helper tools?

• What is it used for? Why do I need it now, even though CocoaPods worked without it before?

• What will happen if I fail to specify some of the files?

• What's the relationship with public_header_files?