Add more information and examples for the log cursors

@sueloverso, I tried to add more information to the log cursor docs, please merge this if you like it.

A few additional items I was unclear on, or thought we should do more:

- The record/operations types overlap and that was confusing to me, they definitely need more documentation if we expect applications to use them. I started to try and write that, but quickly lost track.

- Auto-generating the record/operations types scared me.  The values can't change or log files would break, so adding new values could break the world. Should we hard-code them instead?

- @michaelcahill, now that applications can see the transaction IDs, should we change it so we don't re-use transaction IDs in the logging case?

- I can't think of a way an application can use the file ID, it's not mapped to anything useful (and in the case of a table on top of several files, it's worse).  Should we even return it?

- I'd suggest changing `WT_LOGREC_MESSAGE` to `WT_LOGREC_APPLICATION`, and make it really obvious what's going on.