Is there a variable where one can store a document version number and show
it on the document? It would be incremented every time the document is
changed and then saved back to disk.
Yes, there is such a variable built in -- it's called the
RevisionNumber, and you can see it in the File > Properties dialog on
the Statistics tab. To show it in the document, insert a RevNum field.
From my point of view, though, it's less than useful. If you work on a
document for more than a few minutes, you ought to save every time you
stop to think about what you're writing -- that way, you'll never lose
more than a little bit of work. My documents routinely acquire
revision numbers in the hundreds.
You can define a custom property (on the Custom tab of the same
dialog) to store a "major version" number, but you'll have to update
it manually when you want it to change.
