docs: improve C-FFI docs, link to corresponding JSON-RPC docs

[r10s]

this PR mainly links to at least some JSON-RPC docs - not sure, if there is a better link, this is the best source i could find :)

at some point, we will streamline all that, but also for now it is good to have some pointers (i am also failing regularly to find json-rpc docs :)

[Simon-Laux]

Or in cases where you can not use the cffi, for example when you need to use the api over a process boundary where you can not safely pass over C-Pointers.
Like in deltachat desktop using the deltachat api from the frontend, which is a separate process.

[r10s]

sure, we created the JSON-RPC for that - but this is about the documentation of C-FFI. but i refined that point

[Simon-Laux]

First of all thanks for caring!

at some point, we will streamline all that, but also for now it is good to have some pointers (i am also failing regularly to find json-rpc docs :)

I have improvements for jsonrpc documentation included in the nlnet proposal that I submitted yesterday ("improve bot docs").
In an ideal world we would have auto generation of a wrapper library also for python, java and swift. To get the same type checking, autocomplete and docs generation that we have for typescript
But that's currently out of scope.

While the description in this pr might sound a bit negative of the jsonrpc api, the one I made in my draft blogpost is a bit biased against the cffi.

I have a section at the end of my blog post draft, mentioning what still needs to be done to improve the jsonrpc api.
I believe I don't have all the aspects yet that need to be improved. For example I just added the naming inconsistency topic to the to do of the pr. (thanks for mentioning it again, it had forgotten about it)
It would be great if you could help me complete the list of jsonrpc shortcomings, that:

• we have a list that we can track
• that I can mention that list in the blogpost under the motto of showing the reader what still needs improvements and invite them to contribute

The blog post draft: deltachat/deltachat-pages#984

[r10s]

yeah, i know there are ongoing efforts, this is why i wrote "we will get to improve that" :) for other points, i commented at deltachat/deltachat-android#3459 (comment)

however, the gist of this PR is not to discuss the pros+cons of C-FFI s. JSON-RPC, but to add a pointer to JSON-RPC and to clarify some things for C-FFI usage as there was just nothing before, and it regularly happens that i get asked :)

so, this PR is even pro-JSON-RPC, if you will, it adds a pointer, also clearly stating that things won't become available outside JSON-RPC :)