Improve docs on `Blob`

[rwb27]

As highlighted by @julianstirling in review, we should:

• * Make sure thing-to-thing use of Blob is clearly explained.
• * Check HTTP use of Blob is correctly documented.
• * Check we don't recommend blob_type any more.
• * Move mention of the internal mechanisms like BlobIOContextDep to developer notes.

[rwb27]

This was mostly done already, but will be closed in an upcoming PR that tweaks the docs.