Clarify which variant of ESC is implemented by `escp.py` and `escpr.py`

[cskau]

The naming of escp.py and escpr.py seems to suggest that they implement 'ESC/P' and 'ESC/P-R' respectively.
However, when looking at the some of the codes in escp.py, e.g. ESC i in _send_line, this does not seem to correspond to the code defined in the "EPSON ESC/P Reference Manual" (June 2004 edition).
The latter defines ESC i as "Select immediate print mode'' under "Deleted commands", listing it as a 9-Pin ESC/P command present in neither ESC/P nor ESC/P 2.