Add JavaDoc from interface to implementation

[MarcelCoding]

https://github.com/springdoc/springdoc-openapi/issues/1548

Add the ability to read JavaDoc from the implementation, where the JavaDoc is actually placed at the interface.

[gebezs]

I spent some time understanding the issue and this is what I found:

• the javadoc is already extracted by the annotation processor
• it can be solved in the therapi-runtime-javadoc module only
• figuring out which method overrides what other or others is hard
  • this SO answer is the best solution I found so far (no library as far as I know)
  • there are a lot of edge cases
  • testing is hard
  • I didn't checked the inter module inheritance (superclass/interface in another jar)
• the strange cases are handled by the javadoc tool, so it can be checked for conform solution
  • two implemented interfaces, same method signature, different javadoc
• {@inheritDoc} could be solved with a little amount of extra work
• Any solution probably would solve this issue also: #62

@dnault What do you think, did I miss anything?

[dnault]

@gebezs Thank you for the detailed analysis.

figuring out which method overrides what other or others is hard

Maybe not impossibly hard. It's hard to find out which methods override a method (as in that StackOverflow question), but I believe the Reflection API provides all the tools necessary to go the other way (given a method, find out which superclass / interface methods it overrides).

@dnault What do you think, did I miss anything?

I think you covered everything! Are you interested in working on this?