Change-Id: I88c272444ca734dc2cbc2592607c11287b90a383

## What changes were proposed in this pull request?

The documentation on File DStreams is enhanced to

1. Detail the exact timestamp logic for examining directories and files.
1. Detail how object stores different from filesystems, and so how using them as a source of data should be treated with caution, possibly publishing data to the store differently (direct PUTs as opposed to stage + rename)

## How was this patch tested?

n/a

Author: Steve Loughran <stevel@hortonworks.com>

Closes #17743 from steveloughran/cloud/SPARK-20448-document-dstream-blobstore.