pgsql: Document that dependency tracking doesn't consider function bodi

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(dot)5792(dot)49827(at)wrigleys(dot)postgresql(dot)org>

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/8df3c7ba74e6800c01a0d2fcd09adf23c298e790

Modified Files
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doc/src/sgml/ddl.sgml | 57 ++++++++++++++++++++++++++++++++++++---------------
1 file changed, 40 insertions(+), 17 deletions(-)