Document cli.default_handler option

[multimeric]

The existence of the cli.default_handler option is documented, and I've established that this has to be set to a function with 1 argument. Unfortunately I'm not sure how to work with this argument. It seems to be a cli_message object, which has no methods.

> str(message)
List of 4
 $ message: chr "cli message alert_warning"
 $ type   : chr "alert_warning"
 $ args   :List of 4
  ..$ text :List of 2
  .. ..$ str   : chr "<FCkNXbE-1v5FCkNXbE-1>: <FCkNXbE-1v6FCkNXbE-1>"
  .. ..$ values:<environment: 0x562267b28480> 
  .. ..- attr(*, "class")= chr "cli_glue_delay"
  ..$ id   : chr "cli-155-1"
  ..$ class: NULL
  ..$ wrap : logi FALSE
 $ pid    : int 155
 - attr(*, "class")= chr [1:2] "cli_message" "condition"
> class(message)
[1] "cli_message" "condition"`

In practice I just want to convert the message to a character vector and then save it somewhere. However this class doesn't seem to have a format() method or anything. Can we please document this class and also the cli.default_handler function?