[R] CRAN incoming checks show NOTE due to internal function which isn't documented

[thisisnic]

Describe the bug, including details regarding any error messages, version, and platform.

From the 13.0.0 release after submission to CRAN:

Flavor: r-devel-linux-x86_64-debian-gcc
Check: Rd \usage sections, Result: NOTE
  Documented arguments not in \usage in documentation object 'register_binding':
    'agg_fun'

While this is on an internal function, let's add a roxygen header with the @rdname tag to register_binding_agg() to stop this triggering a NOTE on CRAN

It appears to be due to this change in R-devel:

‘tools::checkDocFiles()’ and ‘tools::checkRdContents()’ now also check internal Rd files by default, but “specially” (ignoring missing documentation of arguments).

Component(s)

R