gh-96526: Clarify format and __format__ docstrings

Objects/longobject.c

@@ -5528,11 +5528,13 @@ int.__format__
 
     format_spec: unicode
     /
+
+Convert to a string according to format_spec.
 [clinic start generated code]*/
 
 static PyObject *
 int___format___impl(PyObject *self, PyObject *format_spec)
-/*[clinic end generated code: output=b4929dee9ae18689 input=e31944a9b3e428b7]*/
+/*[clinic end generated code: output=b4929dee9ae18689 input=d5e1254a47e8d1dc]*/
 {
     _PyUnicodeWriter writer;
     int ret;

Objects/typeobject.c

@@ -5824,11 +5824,13 @@ object.__format__
   /
 
 Default object formatter.
+
+Return str(self) if format_spec is empty. Raise TypeError otherwise.
 [clinic start generated code]*/
 
 static PyObject *
 object___format___impl(PyObject *self, PyObject *format_spec)
-/*[clinic end generated code: output=34897efb543a974b input=7c3b3bc53a6fb7fa]*/
+/*[clinic end generated code: output=34897efb543a974b input=b94d8feb006689ea]*/
 {
     /* Issue 7994: If we're converting to a string, we
        should reject format specifications */

Python/bltinmodule.c

@@ -677,16 +677,19 @@ format as builtin_format
     format_spec: unicode(c_default="NULL") = ''
     /
 
-Return value.__format__(format_spec)
+Return type(value).__format__(value, format_spec)
 
-format_spec defaults to the empty string.
-See the Format Specification Mini-Language section of help('FORMATTING') for
-details.
+Many built-in types implement format_spec according to the
+Format Specification Mini-language. See help('FORMATTING').
+
+If type(value) does not supply a method named __format__
+and format_spec is empty, then str(value) is returned.
+See also help('SPECIALMETHODS').
 [clinic start generated code]*/
 
 static PyObject *
 builtin_format_impl(PyObject *module, PyObject *value, PyObject *format_spec)
-/*[clinic end generated code: output=2f40bdfa4954b077 input=88339c93ea522b33]*/
+/*[clinic end generated code: output=2f40bdfa4954b077 input=45ef3934b86d5624]*/
 {
     return PyObject_Format(value, format_spec);
 }