Document that dependency tracking doesn't consider function bodies.

If there's anyplace in our SGML docs that explains this behavior, I can't
find it right at the moment.  Add an explanation in "Dependency Tracking"
which seems like the authoritative place for such a discussion.  Per
gripe from Michelle Schwan.

While at it, update this section's example of a dependency-related
error message: they last looked like that in 8.3.  And remove the
explanation of dependency updates from pre-7.3 installations, which
is probably no longer worth anybody's brain cells to read.

The bogus error message example seems like an actual documentation bug,
so back-patch to all supported branches.

Discussion: <20160620160047.5792.49827@wrigleys.postgresql.org>