[Rd] Mention L suffix in integer man page?

[Thomas J. Leeper]

This is just a suggestion for a documentation improvement. I went to `?
integer` (src/library/base/man/integer.Rd) expecting to find an explanation
of the L suffix described in:
https://cran.r-project.org/doc/manuals/R-lang.html#Constants and `?
NumericConstants` but it's not mentioned anywhere. It might be useful to
have:

1. An example of explicit declaration of integers, like the one from `?
NumericConstants`:

## If you actually wanted integers, use an "L" suffix.
class(2L)
typeof(2L)

## These are equal but not identical
2 == 2L
identical(2, 2L)

2. A cross-reference to the `? NumericConstants` man page and/or a
cross-reference to the `? ':'` or `? seq` man pages, which describe how
particularities of how integer sequences can be (reliably) constructed.

Again, just a suggestion.

-Thomas

Thomas J. Leeper