Document list of exceptions from joining qenv objects

[pawelru]

Originally posted by @pawelru in #66 (comment)

• make it publicly available (not a private fun docs)
• make it once only not to maintain multiple places

[donyunardi]

Be sure to include comments from @nikolas-burkoff
#56 (comment)
#56 (comment)
#56 (comment)

The goal is to make user aware of the exceptions and how to handle them.

[donyunardi]

Reading through the PR comment, I believe the original request is to combine the roxygen documentation for .check_joinable function and join method into one area that is public.

We also have pre-defined rules:

1. all private functions should have @keywords internal.
2. private functions shouldn't be exposed in the public docs (to avoid users using private functions).
3. internal functions not listed in pkgdown.

If we're keeping .check_joinable as internal, then how about refer the roxygen documentation for .check_joinable to the join method.

#' If two `qenv` can be joined
#'
#' Checks if two `qenv` objects can be combined.
#' For more information, please read `?join`
#'
#' @param x (`qenv`)
#' @param y (`qenv`)
#' @return `TRUE` if able to join or `character` used to print error message.
#' @keywords internal
.check_joinable <- function(x, y) {

Then under join method (public because it's exported), we explain everything.
@gogonzo what do you think?

[gogonzo]

@donyunardi I agree