Improve some C API documentation (GH-108768)

* Express functions which take argument as a C string in terms of
  functions which take Python object.
* Use "note" directive for PyMapping_HasKey() and
  PyMapping_HasKeyString() notes.

Doc/c-api/object.rst

@@ -43,15 +43,15 @@ Object Protocol
 
 .. c:function:: int PyObject_HasAttrString(PyObject *o, const char *attr_name)
 
-   Returns ``1`` if *o* has the attribute *attr_name*, and ``0`` otherwise.  This
-   is equivalent to the Python expression ``hasattr(o, attr_name)``.  This function
-   always succeeds.
+   This is the same as :c:func:`PyObject_HasAttr`, but *attr_name* is
+   specified as a :c:expr:`const char*` UTF-8 encoded bytes string,
+   rather than a :c:expr:`PyObject*`.
 
    .. note::
 
       Exceptions that occur when this calls :meth:`~object.__getattr__` and
-      :meth:`~object.__getattribute__` methods or while creating the temporary :class:`str`
-      object are silently ignored.
+      :meth:`~object.__getattribute__` methods or while creating the temporary
+      :class:`str` object are silently ignored.
       For proper error handling, use :c:func:`PyObject_GetOptionalAttrString`
       or :c:func:`PyObject_GetAttrString` instead.
 
@@ -68,9 +68,9 @@ Object Protocol
 
 .. c:function:: PyObject* PyObject_GetAttrString(PyObject *o, const char *attr_name)
 
-   Retrieve an attribute named *attr_name* from object *o*. Returns the attribute
-   value on success, or ``NULL`` on failure. This is the equivalent of the Python
-   expression ``o.attr_name``.
+   This is the same as :c:func:`PyObject_GetAttr`, but *attr_name* is
+   specified as a :c:expr:`const char*` UTF-8 encoded bytes string,
+   rather than a :c:expr:`PyObject*`.
 
    If the missing attribute should not be treated as a failure, you can use
    :c:func:`PyObject_GetOptionalAttrString` instead.
@@ -93,15 +93,9 @@ Object Protocol
 
 .. c:function:: int PyObject_GetOptionalAttrString(PyObject *obj, const char *attr_name, PyObject **result);
 
-   Variant of :c:func:`PyObject_GetAttrString` which doesn't raise
-   :exc:`AttributeError` if the attribute is not found.
-
-   If the attribute is found, return ``1`` and set *\*result* to a new
-   :term:`strong reference` to the attribute.
-   If the attribute is not found, return ``0`` and set *\*result* to ``NULL``;
-   the :exc:`AttributeError` is silenced.
-   If an error other than :exc:`AttributeError` is raised, return ``-1`` and
-   set *\*result* to ``NULL``.
+   This is the same as :c:func:`PyObject_GetOptionalAttr`, but *attr_name* is
+   specified as a :c:expr:`const char*` UTF-8 encoded bytes string,
+   rather than a :c:expr:`PyObject*`.
 
    .. versionadded:: 3.13
 
@@ -129,10 +123,9 @@ Object Protocol
 
 .. c:function:: int PyObject_SetAttrString(PyObject *o, const char *attr_name, PyObject *v)
 
-   Set the value of the attribute named *attr_name*, for object *o*, to the value
-   *v*. Raise an exception and return ``-1`` on failure;
-   return ``0`` on success.  This is the equivalent of the Python statement
-   ``o.attr_name = v``.
+   This is the same as :c:func:`PyObject_SetAttr`, but *attr_name* is
+   specified as a :c:expr:`const char*` UTF-8 encoded bytes string,
+   rather than a :c:expr:`PyObject*`.
 
    If *v* is ``NULL``, the attribute is deleted, but this feature is
    deprecated in favour of using :c:func:`PyObject_DelAttrString`.
@@ -158,8 +151,9 @@ Object Protocol
 
 .. c:function:: int PyObject_DelAttrString(PyObject *o, const char *attr_name)
 
-   Delete attribute named *attr_name*, for object *o*. Returns ``-1`` on failure.
-   This is the equivalent of the Python statement ``del o.attr_name``.
+   This is the same as :c:func:`PyObject_DelAttr`, but *attr_name* is
+   specified as a :c:expr:`const char*` UTF-8 encoded bytes string,
+   rather than a :c:expr:`PyObject*`.
 
 
 .. c:function:: PyObject* PyObject_GenericGetDict(PyObject *o, void *context)