Improved the readability of the Sub-documents section of documentation.

[giancab25]

I was using your library for my project, and I noticed some typos in the documentation, specifically in the "Sub-documents" section. This is a very useful tool so I want to make sure everyone can understand how to use it. The "Sub-documents" section is the only place where I made edits, but if you would like someone to look over the rest then I would be happy to help! If I misinterpreted what you originally meant, please let me know.