Adjust the description of OutputPluginCallbacks in pg-doc

Hi,

When I was reading the "Logical Decoding Output Plugins" chapter in pg-doc [1],
I think in the summary section, only the callback message_cb is not described
whether it is required or optional, and the description of callback
stream_prepare_cb seems inaccurate.

And after the summary section, I think only the callback stream_xxx_cb section
and the callback truncate_cb section are not described this tag (are they
required or optional).

I think we could improve these to be more reader friendly. So I tried to write
a patch for these and attach it.

Any thoughts?

Regards,
Wang Wei

[1] - https://www.postgresql.org/docs/devel/logicaldecoding-output-plugin.html