Document caller_arg() limitation in print()

[DanChaltiel]

Hi,

I stumbled upon the caller_arg() limitation when redefining a print method for hours without understanding, and thought it might deserve a bit of documentation.

As stated in https://stackoverflow.com/a/4963041/3888000:

This is a rather special case, as R substitutes foo by its value before calling print when you type the name at the command line.
[...]
There is no way on earth you'll extract the name of the object from anywhere. It's simply not there in the callstack.

Here is a rlang specific reprex:

my_obj=1
class(my_obj)="bar"
print.bar = function(x) rlang::caller_arg(x)
mean.bar = function(x) rlang::caller_arg(x)
my_obj
#> x
mean(my_obj)
#> [1] "my_obj"

#however, this works as intended
print(my_obj)
#> [1] "my_obj"

^{Created on 2024-02-21 with reprex v2.0.2}