Doc improvement: describe `ref` in `install_gitlab` help

[dmurdoch]

The help for install_gitlab contains this:

https://github.com/r-lib/remotes/blob/9355549a6ca30ca5879b63533013ac6d3cd381b0/R/install-gitlab.R#L9-L10

but it doesn't define what ref can be. I imagine the same things as for the ref arg to install_git and related functions are allowed, i.e.

#' @param ref Name of branch, tag or SHA reference to use, if not HEAD.

I'd suggest including that definition in the install_gitlab help page, or updating the install_gitlab code itself to be similar to install_github, with the separate ref parameter.