Update rule about using custom downloaders with more information

## What New Or Updated Would You Like To See?

Add documentation for the rule CPMR0055 to detail why the rule about using custom downloaders was flagged, was the recommended solution is and why it was flagged.

## Why Is It Needed?

We should avoid having stubbed out documentation and instead try including information about the rules.

## Additional Context?

N/A

Search the documentation for rule CPMR0055 and identify where its current stub appears. Add the reason custom downloaders are flagged, the recommended solution, and the rationale for that recommendation; done means the rule has useful explanatory documentation instead of a stub.

Written by the indexing model from the issue text.