`remote`: document changes & new commands

[preminger]

Once these issues, relating to reorganizing the functionality under the remote, have been closed —

• sylabs/singularity#1893
• sylabs/singularity#1894
• sylabs/singularity#1895
• sylabs/singularity#1896

— the documentation in https://docs.sylabs.io/guides/latest/user-guide/endpoint.html should be modified & reorganized accordingly.

Discussion: sylabs/singularity#1639