[Rd] Comments not documented in language reference (PR#2367)

[dmurdoch@pair.com]

The R Language Definition manual should document comments.  I think
this should go after section 10.1 "The Parsing Process" and before
10.2 "Tokens".  Here's some draft text.

@node Comments, Tokens, The parsing process, Parser
@comment  node-name,  next,  previous,  up
@section Comments

Comments in R are ignored by the parser.  Any text from a \# character
to the end of the line is taken to be a comment.  For example, 

@smallexample
> x <- 1  # This is a comment
@smallexample