in the shell import pydoc and browse it. Problem is it locks Poser until you quit the browser. The HTML page remains available though.

Most common problem with old scripts available on code (.py)  level is that 'print' now requires its arguments between brackets. Most PoserPython scripts available on source code level work if you add the brackets and make a fix here and there. Also I've seen scripts where some 'clever' use of strings is no longer supported.

Compiled scripts suffer from the 'magic number' issue. Sometimes decompiled code just runs, or does so after 'print brackets' was solved.

Last but not least there's often access rights issues with old scripts. Today a PC is no longer a personal computer. 

But if there were a 'Poser 13 Python Methods Manual' it would not be very different from same for Poser 11, except for the additions and corrections.