Refer to strftime(3) manpage for platform specific format codes.

Suggested by Skip Montanaro on docs@.

Doc/library/datetime.rst

@@ -1603,7 +1603,8 @@ values.  If they're used anyway, ``0`` is substituted for them.
 
 The full set of format codes supported varies across platforms, because Python
 calls the platform C library's :func:`strftime` function, and platform
-variations are common.
+variations are common.  To see the full set of format codes supported on your
+platform, consult the :manpage:`strftime(3)` documentation.
 
 The following is a list of all the format codes that the C standard (1989
 version) requires, and these work on all platforms with a standard C

Doc/library/time.rst

@@ -350,8 +350,10 @@ The module defines the following functions and data items:
       >>> strftime("%a, %d %b %Y %H:%M:%S +0000", gmtime())
       'Thu, 28 Jun 2001 14:17:15 +0000'
 
-   Additional directives may be supported on certain platforms, but only the ones
-   listed here have a meaning standardized by ANSI C.
+   Additional directives may be supported on certain platforms, but only the
+   ones listed here have a meaning standardized by ANSI C.  To see the full set
+   of format codes supported on your platform, consult the :manpage:`strftime(3)`
+   documentation.
 
    On some platforms, an optional field width and precision specification can
    immediately follow the initial ``'%'`` of a directive in the following order;