Explain adding controls for new sandbox types

[aaaaalbert]

When the nodemanager is amended to include new sandbox types besides Repy, the experiment management tools (seash, experiment lib) need to accomodate them as well. This essentially requires changes to the start_target function, and the user-facing command bindings (https://github.com/TsumikiFreederation/seash/blob/master/command_callbacks.py#L2164, https://github.com/TsumikiFreederation/seash/blob/master/command_callbacks.py#L2215).

The docstrings need to document what command should be sent to the remote nodemanagers to start what type of sandbox, how auto-recognition of sandbox types is implemented for Repy, etc.