trac 16xxx correct a bad formatting of doc in morphism.pyx + more doc
trac #16696 reviewer's comments
Trac #16696: correct a typo in doc of morphism.pyx

I have found a typo there. Let us correct this now.

Taking the opportunity to enhance the coverage also.

URL: http://trac.sagemath.org/16696
Reported by: chapoton
Ticket author(s): Frédéric Chapoton
Reviewer(s): Peter Bruin