bpo-29549: Fixes docstring for str.index (#256)

* Updates B.index documentation.

* Updates str.index documentation, makes it Argument Clinic compatible.

* Removes ArgumentClinic code.

* Finishes string.index documentation.

* Updates string.rindex documentation.

* Documents B.rindex.

Objects/unicodeobject.c

@@ -11697,7 +11697,11 @@ unicode_hash(PyObject *self)
 PyDoc_STRVAR(index__doc__,
              "S.index(sub[, start[, end]]) -> int\n\
 \n\
-Like S.find() but raise ValueError when the substring is not found.");
+Return the lowest index in S where substring sub is found, \n\
+such that sub is contained within S[start:end].  Optional\n\
+arguments start and end are interpreted as in slice notation.\n\
+\n\
+Raises ValueError when the substring is not found.");
 
 static PyObject *
 unicode_index(PyObject *self, PyObject *args)
@@ -12813,7 +12817,11 @@ unicode_rfind(PyObject *self, PyObject *args)
 PyDoc_STRVAR(rindex__doc__,
              "S.rindex(sub[, start[, end]]) -> int\n\
 \n\
-Like S.rfind() but raise ValueError when the substring is not found.");
+Return the highest index in S where substring sub is found,\n\
+such that sub is contained within S[start:end].  Optional\n\
+arguments start and end are interpreted as in slice notation.\n\
+\n\
+Raises ValueError when the substring is not found.");
 
 static PyObject *
 unicode_rindex(PyObject *self, PyObject *args)