Clarifying whether write/erase should be a "flush"

[jamesmunns]

Right now, the write/erase methods of NorFlash (and friends) don't clarify whether they guarantee that the actual operation has COMPLETED or not.

Today I was writing an impl for the mx25r chip (here, and @diondokter pointed out that I was doing my "wait not busy" loops BEFORE doing operations, instead of after. This is nice, because we can move on as soon as the data/command is sent.

HOWEVER, if a user expects that write/erase are COMPLETE when the function returns, they might immediately shut down the external flash, even though the write/erase operation is still ongoing.

IMO, we should EITHER:

• Update the docs to say that the existing methods should only return when the operation is COMPLETE, e.g. you should spin on the busy bit after performing the operation, OR
• Add a flush method (this is probably a breaking change), that allows users to separate that when necessary.