Fix code comments regarding logical decoding

Back in 3b02ea4f I added some comments in various places to explain
how logical decoding and other things worked.  Not all of the changes
were welcome, because they were misleading or wrong.  This changes them
a little bit to make them more accurate.

Some other comments are also changed to be more accurate.  Also, fix a
bunch of typos.

Author: Álvaro Herrera, Craig Ringer

Andres Freund reviewed some parts of this.