[docs] datetime.strftime nits

[Skip Montanaro]

How about this modification? From this:

The full set of format codes supported varies across platforms,
because Python calls the platform C library’s strftime()function, and
platform variations are common.

to this:

The full set of format codes supported varies across platforms,
because Python calls the platform C library’s strftime()function, and
platform variations are common. To see the full set of format codes
supported on your platform, consult the strftime(3) documentation.

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