period 'last/first' specification is not explicitely defined in the documentation

[jypeter]

The fact that yon can specify last_100y or FIRST_5y and some variations is shown in some notebook examples and in some parts of the documentation, but it should probably be added to the init_period documentation

I have used the search function of the documentation, and the best I have found was in a source code (match = re.match("(?P<option>last|LAST|first|FIRST)_(?P<duration>[0-9]*)([yY])$", kwargs['period']))