blob: clearly document cases where blob saves a Context for later use

[vangent]

In at least one case (NewWriter), blob saves a context for later use.

• ctx is passed to NewWriter
• If new opt.ContentType is provided, it saves the ctx in the returned *Writer.
• That ctx may be used later as part of a call to Write or to Close (which don't take a Context argument).

The Context documentation says "Do not store Contexts inside a struct type", so this is kind of a no-no, but I'm not sure how to avoid it. There's a proposal to add Context args to Write and Close, but that's speculative and unlikely to happen soon.

We should at least clearly document the behavior.