DOC: Use 'RE.md =' instead of 'RE = RunEngine(...)'
DOC: Use 'RE.md =' instead of 'RE = RunEngine(...)'

- Use `RE.md = ...` in all examples (instead of creating a fresh RE instance).
- Document that RunEngine(md) is possible but put it in a warning box with a
  a warning about "data loss" and a link to the section of the tutorial that
  explains this issue.
- Copy-editing
- Improved formatting

Thanks to @jrmlhermitte for flagging the potential for costly confusion here.

ericdill/bluesky,ericdill/bluesky