sql/builtins: update Info for pg_get_viewdef

The pg_builtin func `pg_get_viewdef` was updated from a no-op to an actual
function long time ago, but the info field is still `notUsableInfo`, which
made the doc to miss it by mistake. This PR is to update the info, and let it
be recorded in `functions.md`.

Release justification: bug fix, update a builtin's visibility in docs.
Release note: none

docs/generated/sql/functions.md

@@ -3457,6 +3457,10 @@ A write probe will effectively probe reads as well.</p>
 </span></td><td>Immutable</td></tr>
 <tr><td><a name="pg_get_serial_sequence"></a><code>pg_get_serial_sequence(table_name: <a href="string.html">string</a>, column_name: <a href="string.html">string</a>) &rarr; <a href="string.html">string</a></code></td><td><span class="funcdesc"><p>Returns the name of the sequence used by the given column_name in the table table_name.</p>
 </span></td><td>Stable</td></tr>
+<tr><td><a name="pg_get_viewdef"></a><code>pg_get_viewdef(view_oid: oid) &rarr; <a href="string.html">string</a></code></td><td><span class="funcdesc"><p>Returns the CREATE statement for an existing view.</p>
+</span></td><td>Stable</td></tr>
+<tr><td><a name="pg_get_viewdef"></a><code>pg_get_viewdef(view_oid: oid, pretty_bool: <a href="bool.html">bool</a>) &rarr; <a href="string.html">string</a></code></td><td><span class="funcdesc"><p>Returns the CREATE statement for an existing view.</p>
+</span></td><td>Stable</td></tr>
 <tr><td><a name="pg_has_role"></a><code>pg_has_role(role: <a href="string.html">string</a>, privilege: <a href="string.html">string</a>) &rarr; <a href="bool.html">bool</a></code></td><td><span class="funcdesc"><p>Returns whether or not the current user has privileges for role.</p>
 </span></td><td>Stable</td></tr>
 <tr><td><a name="pg_has_role"></a><code>pg_has_role(role: oid, privilege: <a href="string.html">string</a>) &rarr; <a href="bool.html">bool</a></code></td><td><span class="funcdesc"><p>Returns whether or not the current user has privileges for role.</p>

pkg/sql/sem/builtins/pg_builtins.go

@@ -273,7 +273,7 @@ func makePGGetViewDef(argTypes tree.ArgTypes) tree.Overload {
 			}
 			return r[0], nil
 		},
-		Info:       notUsableInfo,
+		Info:       "Returns the CREATE statement for an existing view.",
 		Volatility: volatility.Stable,
 	}
 }