Improve documentation about binary/textual output mode for output plugins.

Andres Freund <andres@anarazel.de>

Commit: 0ef3c29a4b43e72d93cff65a17a9ccccff87618d
Author: Andres Freund <andres@anarazel.de>
Date: 2014-10-01T11:22:17Z
Releases: 9.5.0
Improve documentation about binary/textual output mode for output plugins.

Also improve related error message as it contributed to the confusion.

Discussion: CAB7nPqQrqFzjqCjxu4GZzTrD9kpj6HMn9G5aOOMwt1WZ8NfqeA@mail.gmail.com,
    CAB7nPqQXc_+g95zWnqaa=mVQ4d3BVRs6T41frcEYi2ocUrR3+A@mail.gmail.com

Per discussion between Michael Paquier, Robert Haas and Andres Freund

Backpatch to 9.4 where logical decoding was introduced.

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