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>

doc/src/sgml/ddl.sgml

@@ -3571,14 +3571,14 @@ ANALYZE measurement;
    To ensure the integrity of the entire database structure,
    <productname>PostgreSQL</productname> makes sure that you cannot
    drop objects that other objects still depend on.  For example,
-   attempting to drop the products table we had considered in <xref
+   attempting to drop the products table we considered in <xref
    linkend="ddl-constraints-fk">, with the orders table depending on
-   it, would result in an error message such as this:
+   it, would result in an error message like this:
 <screen>
 DROP TABLE products;
 
-NOTICE:  constraint orders_product_no_fkey on table orders depends on table products
 ERROR:  cannot drop table products because other objects depend on it
+DETAIL:  constraint orders_product_no_fkey on table orders depends on table products
 HINT:  Use DROP ... CASCADE to drop the dependent objects too.
 </screen>
    The error message contains a useful hint: if you do not want to
@@ -3589,11 +3589,12 @@ DROP TABLE products CASCADE;
    and all the dependent objects will be removed.  In this case, it
    doesn't remove the orders table, it only removes the foreign key
    constraint.  (If you want to check what <command>DROP ... CASCADE</> will do,
-   run <command>DROP</> without <literal>CASCADE</> and read the <literal>NOTICE</> messages.)
+   run <command>DROP</> without <literal>CASCADE</> and read the
+   <literal>DETAIL</> output.)
   </para>
 
   <para>
-   All drop commands in <productname>PostgreSQL</productname> support
+   All <command>DROP</> commands in <productname>PostgreSQL</> support
    specifying <literal>CASCADE</literal>.  Of course, the nature of
    the possible dependencies varies with the type of the object.  You
    can also write <literal>RESTRICT</literal> instead of
@@ -3605,21 +3606,43 @@ DROP TABLE products CASCADE;
    <para>
     According to the SQL standard, specifying either
     <literal>RESTRICT</literal> or <literal>CASCADE</literal> is
-    required.  No database system actually enforces that rule, but
-    whether the default behavior is <literal>RESTRICT</literal> or
-    <literal>CASCADE</literal> varies across systems.
+    required in a <command>DROP</> command.  No database system actually
+    enforces that rule, but whether the default behavior
+    is <literal>RESTRICT</literal> or <literal>CASCADE</literal> varies
+    across systems.
    </para>
   </note>
 
-  <note>
-   <para>
-    Foreign key constraint dependencies and serial column dependencies
-    from <productname>PostgreSQL</productname> versions prior to 7.3
-    are <emphasis>not</emphasis> maintained or created during the
-    upgrade process.  All other dependency types will be properly
-    created during an upgrade from a pre-7.3 database.
-   </para>
-  </note>
+  <para>
+   For user-defined functions, <productname>PostgreSQL</productname> tracks
+   dependencies associated with a function's externally-visible properties,
+   such as its argument and result types, but <emphasis>not</> dependencies
+   that could only be known by examining the function body.  As an example,
+   consider this situation:
+
+<programlisting>
+CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow',
+                             'green', 'blue', 'purple');
+
+CREATE TABLE my_colors (color rainbow, note text);
+
+CREATE FUNCTION get_color_note (rainbow) RETURNS text AS
+  'SELECT note FROM my_colors WHERE color = $1'
+  LANGUAGE SQL;
+</programlisting>
+
+   (See <xref linkend="xfunc-sql"> for an explanation of SQL-language
+   functions.)  <productname>PostgreSQL</productname> will be aware that
+   the <function>get_color_note</> function depends on the <type>rainbow</>
+   type: dropping the type would force dropping the function, because its
+   argument type would no longer be defined.  But <productname>PostgreSQL</>
+   will not consider <function>get_color_note</> to depend on
+   the <structname>my_colors</> table, and so will not drop the function if
+   the table is dropped.  While there are disadvantages to this approach,
+   there are also benefits.  The function is still valid in some sense if the
+   table is missing, though executing it would cause an error; creating a new
+   table of the same name would allow the function to work again.
+  </para>
  </sect1>
 
 </chapter>