add :doc metadata to the R functions generated

[behrica]

I made a proof-of-concept on for this and it does work in principle.
We can reuse the "help" code which creates and formats the help and attach to the var "meta" under key :doc.

The proof of concept is here:
https://github.com/behrica/clojuress/tree/helpToDoc

so we don't loose it

[behrica]

see here:
https://clojurians.zulipchat.com/#narrow/stream/224816-clojisr-dev/topic/get.20documentation.20of.20R.20functions.20in.20IDE

the above code works, but "copies" instead of re-uses the help code. Fixing this needed code-reorganisation to avoid circular dependencies

[behrica]

PR in progress #98