Improve documentation, explaining how affinder works

[jni]

This imagesc thread asks some very fundamental questions about affinder that should be answered in the docs:

https://forum.image.sc/t/affinder-support-request/79861

specifically:

• where is the affinder data stored "in flight"
• how can affinder be integrated into a larger workflow
• how can we monitor residuals and edit point pairs without messing up affinder's callbacks too much