What documentation exists for R?

Chapter 6: R and Emacs 78
To specify command line arguments for the inferior R process, use C-u M-x R for starting
R.
6.3 Debugging R from within Emacs
To debug R “from within Emacs”, there are several possibilities. To use the Emacs GUD
(Grand Unified Debugger) library with the recommended debugger GDB, type M-x gdb
and give the path to the R binary as argument. At the gdb prompt, set R_HOME and
other environment variables as needed (using e.g. set env R_HOME /path/to/R/, but see
also below), and start the binary with the desired arguments (e.g., run --quiet).
If you have ESS, you can do C-u M-x R 〈RET〉 - d 〈SPC〉 g d b 〈RET〉 to start an inferior R
process with arguments ‘-d gdb’.
A third option is to start an inferior R process via ESS (M-x R) and then start GUD
(M-x gdb) giving the R binary (using its full path name) as the program to debug. Use the
program ps to find the process number of the currently running R process then use the
attach command in gdb to attach it to that process. One advantage of this method is that
you have separate *R* and *gud-gdb* windows. Within the *R* window you have all the
ESS facilities, such as object-name completion, that we know and love.
When using GUD mode for debugging from within Emacs, you may find it most convenient
to use the directory with your code in it as the current working directory and then
make a symbolic link from that directory to the R binary. That way ‘.gdbinit’ can stay
in the directory with the code and be used to set up the environment and the search paths
for the source, e.g. as follows:
set env R_HOME /opt/R
set env R_PAPERSIZE letter
set env R_PRINTCMD lpr
dir /opt/R/src/appl
dir /opt/R/src/main
dir /opt/R/src/nmath
dir /opt/R/src/unix