good source for explaining input and output parameters of R functions

It's almost entirely up to the author of the function how much (or little)
to document in the help page.  Packages on CRAN go through some checks, one
of which is that all parameters of functions are documented in the help
page.  However, the return value is another matter.  It's supposed to be
documented in the "Value" section of the help page, but some people don't
bother documenting everything being returned, sometimes because some of them
weren't meant to be used by user directly.

When all else fails, read the R code itself.

HTH,
Andy