Improve documentation about our XML functionality.

Add a section explaining how our XML features depart from current
versions of the SQL standard.  Update and clarify the descriptions
of some XML functions.

Chapman Flack, reviewed by Ryan Lambert

Discussion: https://postgr.es/m/5BD1284C.1010305@anastigmatix.net
Discussion: https://postgr.es/m/5C81F8C0.6090901@anastigmatix.net
Discussion: https://postgr.es/m/CAN-V+g-6JqUQEQZ55Q3toXEN6d5Ez5uvzL4VR+8KtvJKj31taw@mail.gmail.com

doc/src/sgml/datatype.sgml

@@ -4219,6 +4219,11 @@ a0ee-bc99-9c0b-4ef8-bb6d-6bb9-bd38-0a11
     value is a full document or only a content fragment.
    </para>
 
+   <para>
+    Limits and compatibility notes for the <type>xml</type> data type
+    can be found in <xref linkend="xml-limits-conformance"/>.
+   </para>
+
    <sect2>
     <title>Creating XML Values</title>
    <para>

src/backend/catalog/sql_features.txt

@@ -593,7 +593,7 @@ X085	Predefined namespace prefixes			NO
 X086	XML namespace declarations in XMLTable			NO	
 X090	XML document predicate			YES	
 X091	XML content predicate			NO	
-X096	XMLExists			NO	XPath only
+X096	XMLExists			NO	XPath 1.0 only
 X100	Host language support for XML: CONTENT option			NO	
 X101	Host language support for XML: DOCUMENT option			NO	
 X110	Host language support for XML: VARCHAR mapping			NO	
@@ -661,11 +661,11 @@ X282	XMLValidate with CONTENT option			NO
 X283	XMLValidate with SEQUENCE option			NO	
 X284	XMLValidate: NAMESPACE without ELEMENT clause			NO	
 X286	XMLValidate: NO NAMESPACE with ELEMENT clause			NO	
-X300	XMLTable			NO	XPath only
+X300	XMLTable			NO	XPath 1.0 only
 X301	XMLTable: derived column list option			YES	
 X302	XMLTable: ordinality column option			YES	
 X303	XMLTable: column default option			YES	
-X304	XMLTable: passing a context item			YES	
+X304	XMLTable: passing a context item			YES	must be XML DOCUMENT
 X305	XMLTable: initializing an XQuery variable			NO	
 X400	Name and identifier mapping			YES	
 X410	Alter column data type: XML type			YES