Explain where distro info comes from

**What would you like to be added**:
Explain where distro info comes from

**Why is this needed**:
We don’t explain where we got distro information from (like we do with catalogers)

**Additional context**:

Shouldn’t the distro identification be a cataloger?
Could the distro output be under “artifacts” like all other cataloger output?

It would help with https://github.com/anchore/syft/issues/691

Start by reading issue #691 and reviewing the existing explanations of cataloger output. Determine where distro information is currently produced, then document its source and resolve whether distro identification belongs to a cataloger and whether its output should appear under artifacts; done means the behavior and placement are clearly explained.

Written by the indexing model from the issue text.