expand Rationale

Author: skirpichev

peps/pep-0791.rst

@@ -66,7 +66,9 @@ Rationale
 Why not fix the :external+py3.14:mod:`math` module documentation instead?
 Sure, we can be much more vague in the module preamble (i.e. roughly say
 that "the :external+py3.14:mod:`math` module contains some mathematical
-functions") and we can accurately describe input/output for each function.
+functions"), we can accurately describe input/output for each function
+and it's behavior (e.g. whether the :external+py3.14:func:`~math.factorial`
+output is exact or not, like e.g. the `scipy.special.factorial <https://docs.scipy.org/doc/scipy/reference/generated/scipy.special.factorial.html#scipy.special.factorial>`_, per default).
 
 But the major issue is that the current module mixes different, almost non-interlaced
 application domains.  Adding more documentation will just highlight this and