This patch adds the ability to run a command in Eshell from any host, no 
matter your current directory. For example, you could run 
"/ssh:user@remote:whoami" from a local dir, which would run "whoami" 
over the SSH connection for "user@remote". Similarly, you could run 
"/:whoami" to run the local "whoami" even from a remote dir. (The latter 
syntax is just piggybacking off of quoted file names, which otherwise 
have no special meaning in Eshell.)

Prior to the main patch, I also added a bit of documentation about how 
remote access works in Eshell. These are separate commits so that, if we 
wanted, we could backport the first patch to the Emacs 29 branch.