Describe hook_inline_conditions_build_alter().

Not knowing the code and the internal structure, I needed to add some Commerce Discount conditions. To me the README was very terse, but good enough. I was confident that I'm looking in the right module, and saw which code to copy & modify.

Where I got stuck after copying code: I didn't know

• if I change an autocomplete element on the configure form, to a select element... then what else should I change?
• where does the data get stored and in what format?
• my rules condition is not OK (has an empty value). what to do about it?
• what does hook_inline_conditions_build_alter() do?

I guess I should be writing a HOWTO now, but I won't :)
The next best thing is documenting hook_inline_conditions_build_alter(). From there it's hopefully clear that the info is stored in 2 places, and flows from submitted configure-form values => inline-conditions field => the hook => rules condition.