Documentation of `PyFile_Write{Object,String}` does not mention other writable targets, only files

[sobolevn]

Feature or enhancement

The docs say:

cpython/Doc/c-api/file.rst

Lines 91 to 98 in ac01e22

	.. c:function:: int PyFile_WriteObject(PyObject *obj, PyObject *p, int flags)
	.. index:: single: Py_PRINT_RAW
	Write object *obj* to file object *p*. The only supported flag for *flags* is
	:c:macro:`Py_PRINT_RAW`; if given, the :func:`str` of the object is written
	instead of the :func:`repr`. Return ``0`` on success or ``-1`` on failure; the
	appropriate exception will be set.

cpython/Doc/c-api/file.rst

Lines 101 to 104 in ac01e22

	.. c:function:: int PyFile_WriteString(const char *s, PyObject *p)
	Write string *s* to file object *p*. Return ``0`` on success or ``-1`` on
	failure; the appropriate exception will be set.

However, implementation is a bit different (here's the important part of PyFile_WriteObject which is used for both funcs):

cpython/Objects/fileobject.c

Lines 119 to 131 in ac01e22

	writer = PyObject_GetAttr(f, &_Py_ID(write));
	if (writer == NULL)
	return -1;
	if (flags & Py_PRINT_RAW) {
	value = PyObject_Str(v);
	}
	else
	value = PyObject_Repr(v);
	if (value == NULL) {
	Py_DECREF(writer);
	return -1;
	}
	result = PyObject_CallOneArg(writer, value);

So, basically any object with one-argument write will work. In my tests I use io.StringIO https://github.com/python/cpython/pull/111709/files#diff-b636fe6ac656d3029788c6c9431c0884dba5316e815ec64c1165597cdee065f8R86 which is clearly not a file.

Other similar docs have this note:

cpython/Doc/c-api/file.rst

Lines 52 to 54 in ac01e22

	Equivalent to ``p.readline([n])``, this function reads one line from the
	object *p*. *p* may be a file object or any object with a
	:meth:`~io.IOBase.readline`

So, I think here we can reference IOBase.write (but it does not exist for some reason):

[terryjreedy]

Docs for Python functions/methods often say 'file-like object' or 'object with write method'.

Something does seem to be missing. The writelines mixin requires stub write. Similarly for readline(s).