Issue #12644: document the '%a' conversion in the old string formatting…

Issue #12644: document the '%a' conversion in the old string formatting operations. Patch prepared together with Ezio Melotti

Doc/library/stdtypes.rst

@@ -1464,7 +1464,7 @@ components, which must occur in this order:
    object to convert comes after the minimum field width and optional precision.
 
 #. Precision (optional), given as a ``'.'`` (dot) followed by the precision.  If
-   specified as ``'*'`` (an asterisk), the actual width is read from the next
+   specified as ``'*'`` (an asterisk), the actual precision is read from the next
    element of the tuple in *values*, and the value to convert comes after the
    precision.
 
@@ -1546,9 +1546,12 @@ The conversion types are:
 | ``'r'``    | String (converts any Python object using            | \(5)  |
 |            | :func:`repr`).                                      |       |
 +------------+-----------------------------------------------------+-------+
-| ``'s'``    | String (converts any Python object using            |       |
+| ``'s'``    | String (converts any Python object using            | \(5)  |
 |            | :func:`str`).                                       |       |
 +------------+-----------------------------------------------------+-------+
+| ``'a'``    | String (converts any Python object using            | \(5)  |
+|            | :func:`ascii`).                                     |       |
++------------+-----------------------------------------------------+-------+
 | ``'%'``    | No argument is converted, results in a ``'%'``      |       |
 |            | character in the result.                            |       |
 +------------+-----------------------------------------------------+-------+
@@ -1581,7 +1584,7 @@ Notes:
    decimal point and defaults to 6.
 
 (5)
-   The precision determines the maximal number of characters used.
+   If precision is ``N``, the output is truncated to ``N`` characters.
 
 
 (7)