bpo-5945: Improve mappings and sequences C API docs. (GH-7029)

diff --git a/Include/abstract.h b/Include/abstract.h
index 3133cd1..4088f75 100644
--- a/Include/abstract.h
+++ b/Include/abstract.h
@@ -442,13 +442,14 @@
    This is the equivalent of the Python statement: o[key]=v. */
 PyAPI_FUNC(int) PyObject_SetItem(PyObject *o, PyObject *key, PyObject *v);
 
-/* Remove the mapping for object, key, from the object 'o'.
+/* Remove the mapping for the string 'key' from the object 'o'.
    Returns -1 on failure.
 
    This is equivalent to the Python statement: del o[key]. */
 PyAPI_FUNC(int) PyObject_DelItemString(PyObject *o, const char *key);
 
-/* Delete the mapping for key from object 'o'. Returns -1 on failure.
+/* Delete the mapping for the object 'key' from the object 'o'.
+   Returns -1 on failure.
 
    This is the equivalent of the Python statement: del o[key]. */
 PyAPI_FUNC(int) PyObject_DelItem(PyObject *o, PyObject *key);
@@ -1005,8 +1006,7 @@
 PyAPI_FUNC(int) PyMapping_Check(PyObject *o);
 
 /* Returns the number of keys in mapping object 'o' on success, and -1 on
-  failure. For objects that do not provide sequence protocol, this is
-  equivalent to the Python expression: len(o). */
+  failure. This is equivalent to the Python expression: len(o). */
 PyAPI_FUNC(Py_ssize_t) PyMapping_Size(PyObject *o);
 
 /* For DLL compatibility */
@@ -1019,7 +1019,7 @@
 
    int PyMapping_DelItemString(PyObject *o, const char *key);
 
-   Remove the mapping for object 'key' from the mapping 'o'. Returns -1 on
+   Remove the mapping for the string 'key' from the mapping 'o'. Returns -1 on
    failure.
 
    This is equivalent to the Python statement: del o[key]. */
@@ -1029,7 +1029,7 @@
 
    int PyMapping_DelItem(PyObject *o, PyObject *key);
 
-   Remove the mapping for object 'key' from the mapping object 'o'.
+   Remove the mapping for the object 'key' from the mapping object 'o'.
    Returns -1 on failure.
 
    This is equivalent to the Python statement: del o[key]. */
@@ -1063,13 +1063,13 @@
    NULL. */
 PyAPI_FUNC(PyObject *) PyMapping_Items(PyObject *o);
 
-/* Return element of o corresponding to the object, key, or NULL on failure.
+/* Return element of 'o' corresponding to the string 'key' or NULL on failure.
 
    This is the equivalent of the Python expression: o[key]. */
 PyAPI_FUNC(PyObject *) PyMapping_GetItemString(PyObject *o,
                                                const char *key);
 
-/* Map the object 'key' to the value 'v' in the mapping 'o'.
+/* Map the string 'key' to the value 'v' in the mapping 'o'.
    Returns -1 on failure.
 
    This is the equivalent of the Python statement: o[key]=v. */