Incorporate gotchas from MFS section of IPFS Camp core course A

[terichadbourne]

This issue is a result of the IPFS Camp content review in #261.

Review gotchas in the MFS section at the end of @alanshaw's Core Course A: Understanding How IPFS Deals with Files from IPFS Camp 2019 ( video | slides | course repo) and add relevant content to:

• the MFS tutorial
• the first lesson of the Files API tutorial where differences between MFS and top-level files methods are discussed