[Rd] R-ints context documentation

[brodie gaslam]

In 1.4 Contexts[1], should the following:

> Note that whilst calls to closures and builtins set a context, 
> those to special internal functions never do.

Be something like:

> Note that whilst calls to closures always set a context,
> those to builtins only set a context under profiling
> or if they are of the foreign variety (e.g `.C` and similar),
> and those to special internal functions never do.

Based on the 'eval.c' source[2].

Best,

Brodie

[1]: https://cran.r-project.org/doc/manuals/r-devel/R-ints.html#Contexts
[2]: https://github.com/wch/r-source/blob/tags/R-4-0-0/src/main/eval.c#L821