remote: clarify which list of references _ls() returns

Make it clear that this is not the ls-remote command but a way to access
the data we have and how long it's kept around.
remote: don't say we free the remote on disconnect

On disconnect we simply ask the transport to close the connection, we do
not free it.
Merge pull request #2717 from libgit2/cmn/remote-ls

Clarify the git_remote_ls() documentation