bpo-12743: Delete comment from marshal.rst (GH-8457)

bpo-12743: Delete comment from marshal.rst (GH-8457)

Also, update the list of exceptions that may raised by PyMarshal_*
functions. We usually don't document exceptions raised by a
function, but in this case most of them were already documented
in C API and standard library documentation.
(cherry picked from commit defcffdf86780e3a184ebb25dc9a7b807753d57a)

Co-authored-by: Berker Peksag <berker.peksag@gmail.com>