Indicate that GPORCA doesn't fully support per-column collation (#7153)

gpdb-doc/dita/admin_guide/query/topics/query-piv-opt-limitations.xml

@@ -48,6 +48,9 @@
             the characters are not compatible with the host system locale.</li>
           <li><codeph>SELECT</codeph>, <codeph>UPDATE</codeph>, and <codeph>DELETE</codeph> commands
             where a table name is qualified by the <codeph>ONLY</codeph> keyword.</li>
+          <li>Per-column collation. GPORCA supports collation only when all columns in the query use
+            the same collation. If columns in the query use different collations, then Greenplum
+            uses the Postgres query planner.</li>
         </ul></p>
     </body>
   </topic>

gpdb-doc/dita/ref_guide/sql_commands/ALTER_TABLE.xml

@@ -164,7 +164,11 @@ where <varname>action</varname> is one of:
                                         default conversion is the same as an assignment cast from
                                         old data type to new. A <codeph>USING</codeph> clause must
                                         be provided if there is no implicit or assignment cast from
-                                        old to new type.</li>
+                                        old to new type.<note>GPORCA supports collation only when
+                                                all columns in the query use the same collation. If
+                                                columns in the query use different collations, then
+                                                Greenplum uses the Postgres query
+                                        planner.</note></li>
                                 <li id="ay136904"><b>SET/DROP DEFAULT</b> — Sets or removes the
                                         default value for a column. The default values only apply to
                                         subsequent <codeph>INSERT</codeph> commands. They do not

gpdb-doc/dita/ref_guide/sql_commands/CREATE_TABLE.xml

@@ -265,7 +265,9 @@ CREATE [ [GLOBAL | LOCAL] {TEMPORARY | TEMP} | UNLOGGED ] TABLE [IF NOT EXISTS]
           <pt>COLLATE <varname>collation</varname></pt>
           <pd>The <codeph>COLLATE</codeph> clause assigns a collation to the column (which must be
             of a collatable data type). If not specified, the column data type's default collation
-            is used.</pd>
+            is used.<note>GPORCA supports collation only when all columns in the query use the same
+              collation. If columns in the query use different collations, then Greenplum uses the
+              Postgres query planner.</note></pd>
         </plentry>
         <plentry>
           <pt>DEFAULT <varname>default_expr</varname></pt>