[3.10] More minor fixes to C API docs (GH-31525) (GH-32258)

* 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.
(cherry picked from commit 897bc6f9282238d5fb32d232ab62d30675244736)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>