More minor fixes to C API docs (GH-31525)

* wording fixes in type.rst

* grammar and punctuation in sys.rst

* set: grammar fixes

* structures: capitalization fix

* grammar fixes for sequence

* objects: point to Py_TYPE instead of direct object access

* numbers: add more explicit Python equivalences

* method: add missing period

* memory: grammar fix

* mapping: grammar fixes

* long: grammar fix

* iter: fix grammar for PyAIter_Check

* init: grammar fix