Update the documentation on elpy-rpc-python-command

[galaunay]

PR Summary

Follow #1713 .

The way the RPC virtualenv is created has been modified lately in #1699.
Elpy now try to use the system python interpreter to create the virtualenv.

It has been done for two reasons:

• virtualenvs created by venv keep a link with the environment they have been created from, which is notably a problem when using pip in them.
• the python interpreters for the RPC and the interactive shells don't have to be the same, so most of the time, it doesn't really matters which interpreter is used for the RPC.

This PR just update the documentation of elpy-rpc-python-command to reflect those changes.

[coveralls]

Coverage increased (+0.1%) to 92.13% when pulling bc9a45a on galaunay:Fix1713 into f679a8a on jorgenschaefer:master.

[coveralls]

Coverage increased (+0.07%) to 92.104% when pulling bc9a45a on galaunay:Fix1713 into f679a8a on jorgenschaefer:master.