ctypes currently has no way to easily create a buffer object
from a pointer and a dynamic size. It is possible to create
memoryview objects of array objects (e.g.
memoryview((c_ubyte * 10)())) but this is excessively slow when
implementing a callback function in Python that is passed a
dynamic void * and a size_t.

`ctypes.buffer_at()` fills that gap in the API. This is similar
to `ffi.buffer()` in the cffi module.

Documentation improvements

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

It's a more descriptive and precise name.

It's more common that the user will want a memoryview object that can
be mutated. An immutable object is more rare, so make the default case
return a mutable object.

…tics. Improve tests and docs.

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>