I was wondering what else common sources available until search the source code. Maybe better to make this clear.

Author: Yijie Shen <henry.yijieshen@gmail.com>

Closes #6641 from yijieshen/patch-1 and squashes the following commits:

b5b99b4 [Yijie Shen] Make it clear that JvmSource is the only available additional source currently
f23140c [Yijie Shen] [DOC][Minor]Specify the common sources available for collecting