Suggestion: include a bit on memory management

[madphysicist]

Some questions that have been bothering me about the functions described in https://docs.python.org/3/c-api/memory.html:

• What are the differences between PyMem_RawMalloc, PyMem_Malloc, PyObject_Malloc, etc.?
• Do these functions automatically raise a MemoryError if they return None?

While the answers are out there (I ended up reading the source code and some Stack Overflow posts), they are non-trivial to find.