Document imprint usage in association files

[stscijgbot-jp]

Issue JP-3811 was created on JIRA by Melanie Clarke:

Update the imprint step documentation to describe how users should include imprint exposures in their processing, as is done for the backgrounds in the Image-from-Image background subtraction page ([https://jwst-pipeline.readthedocs.io/en/latest/jwst/background_subtraction/main.html#image-from-image-subtraction]). Currently the documentation for the imprint step doesn't mention that leakcal exposures should be added to ASNs as "imprint" members.