Fix documentation for the --python option
Fix documentation for the --python option

The default value differs for asv run and asv dev.  Moreover, currently
only python=same results to use of ExistingEnvironment, so it's not
possible to pass in python executable names any more.

pv/asv,pv/asv,airspeed-velocity/asv,qwhelan/asv,qwhelan/asv,spacetelescope/asv,spacetelescope/asv,spacetelescope/asv,qwhelan/asv,edisongustavo/asv,mdboom/asv,spacetelescope/asv,giltis/asv,giltis/asv,mdboom/asv,mdboom/asv,edisongustavo/asv,airspeed-velocity/asv,airspeed-velocity/asv,pv/asv,qwhelan/asv,pv/asv,edisongustavo/asv,airspeed-velocity/asv,giltis/asv,mdboom/asv