Update documentation publishing instructions

[apjanke]

The documentation publishing instructions look a little out of date. This is a suggestion on fixing them up.

This changes:

• Removes reference to nonexistent javadoc directory. Maybe this is a leftover from an older build process?
• Checks out the version tag before building the javadoc, to make sure they're getting exactly that version, and no subsequent changes on master.
• Includes steps to update the latest symlink.
• Includes a commit comment that matches the format I saw in the gh-pages branch history.