Document mpgen

[Sjors]

It would be useful to document mpgen in the Design doc and/or elsewhere.

It could include some remarks about cross-compilation, see e.g. #171. IIUC mpgen is only needed at compile time, so doesn't need to be (and shouldn't be) cross-compiled itself.

[Sjors]

It may also be useful to point to TargetCapnpSources.cmake in the design doc, because it seems to be where much of the action is.

[ryanofsky]

Agree, it would be good to clean up the design and usage docs:

• https://github.com/bitcoin-core/libmultiprocess/blob/master/doc/usage.md
• https://github.com/bitcoin-core/libmultiprocess/blob/master/doc/design.md

and add more information about mpgen.

The bitcoin design doc probably also has an explanation of mpgen: https://github.com/bitcoin/bitcoin/blob/master/doc/design/multiprocess.md#the-mpgen-code-generation-tool and a "Diagram showing generated source files and includes" (captioned as such). But it looks like the diagram is actually in the wrong section of the document and should be moved above closer to the descriptions of generated code.

Relatedly, there is another diagram of generated files bitcoin/bitcoin#30510 (comment) that should probably be incorporated somewhere.

Since it's been some time, it would probably be good to look at the existing documentation with fresh eyes and see if it could be organized and written better, but in the meantime any incremental improvements like adding links between the documents or mentioning things like cmake & cross compilation details should also be welcome.