Rewrite window function reference section to more clearly explain

keywords and concepts, based on suggestions by Florian Pflug.

doc/src/sgml/syntax.sgml

@@ -1742,11 +1742,16 @@ UNBOUNDED FOLLOWING
    <para>
     Here, <replaceable>expression</replaceable> represents any value
     expression that does not itself contain window function calls.
-    The <literal>PARTITION BY</> and <literal>ORDER BY</> lists have
-    essentially the same syntax and semantics as <literal>GROUP BY</>
-    and <literal>ORDER BY</> clauses of the whole query, except that their
+    <literal>PARTITION BY</> behaves like a <literal>GROUP
+    BY</> clause by grouping rows into partitions, except its
     expressions are always just expressions and cannot be output-column
-    names or numbers.
+    names or numbers.   <literal>ORDER BY</> behaves similar to a
+    query-level <literal>ORDER BY</> clause, except it controls the
+    order rows are supplied to the aggregate function within the window
+    frame, and with the same expression restrictions.
+   </para>
+
+   <para>
     <replaceable>window_name</replaceable> is a reference to a named window
     specification defined in the query's <literal>WINDOW</literal> clause.
     Named window specifications are usually referenced with just
@@ -1760,39 +1765,48 @@ UNBOUNDED FOLLOWING
     page for details.
    </para>
 
+   <para>
+    In <literal>RANGE</> mode, <literal>CURRENT ROW</> starts with
+    the current row's first peer that <literal>ORDER BY</>
+    considers equivalent, and ends with its last equivalent peer.  In 
+    <literal>ROWS</> mode, <literal>CURRENT ROW</> simply starts and ends
+    with the current row.
+   </para>
+
+   <para>
+    <literal>UNBOUNDED PRECEDING</> means that the frame
+    starts with the first row of the partition, and similarly
+    <literal>UNBOUNDED FOLLOWING</> means that the frame ends with the last
+    row of the partition.
+    The <replaceable>value</> <literal>PRECEDING</> and
+    <replaceable>value</> <literal>FOLLOWING</> cases are currently only
+    allowed in <literal>ROWS</> mode.  They indicate that the frame starts
+    or ends the specified number of rows before or after the current row.
+    <replaceable>value</replaceable> must be an integer expression not
+    containing any variables, aggregate functions, or window functions.
+    The value must not be null or negative; but it can be zero, which
+    just selects the current row.
+   </para>
+
    <para>
     The <replaceable class="parameter">frame_clause</replaceable> specifies
     the set of rows constituting the <firstterm>window frame</>, for those
     window functions that act on the frame instead of the whole partition.
-    If <replaceable>frame_end</> is omitted it defaults to <literal>CURRENT
-    ROW</>.  Restrictions are that
+    The default framing option is <literal>RANGE UNBOUNDED PRECEDING</>,
+    which is the same as <literal>RANGE BETWEEN UNBOUNDED PRECEDING AND
+    CURRENT ROW</>.  This sets the frame to be all rows from the partition start
+    up through the current row's last peer as specified by <literal>ORDER BY</>,
+    or all partition rows if there is no <literal>ORDER BY</>.
+   </para>
+
+   <para>
+    Restrictions are that
     <replaceable>frame_start</> cannot be <literal>UNBOUNDED FOLLOWING</>,
     <replaceable>frame_end</> cannot be <literal>UNBOUNDED PRECEDING</>,
     and the <replaceable>frame_end</> choice cannot appear earlier in the
     above list than the <replaceable>frame_start</> choice &mdash; for example
     <literal>RANGE BETWEEN CURRENT ROW AND <replaceable>value</>
     PRECEDING</literal> is not allowed.
-    The default framing option is <literal>RANGE UNBOUNDED PRECEDING</>,
-    which is the same as <literal>RANGE BETWEEN UNBOUNDED PRECEDING AND
-    CURRENT ROW</>; it sets the frame to be all rows from the partition start
-    up through the current row's last peer in the <literal>ORDER BY</>
-    ordering (which means all rows if there is no <literal>ORDER BY</>).
-    In general, <literal>UNBOUNDED PRECEDING</> means that the frame
-    starts with the first row of the partition, and similarly
-    <literal>UNBOUNDED FOLLOWING</> means that the frame ends with the last
-    row of the partition (regardless of <literal>RANGE</> or <literal>ROWS</>
-    mode).  In <literal>ROWS</> mode, <literal>CURRENT ROW</>
-    means that the frame starts or ends with the current row; but in
-    <literal>RANGE</> mode it means that the frame starts or ends with
-    the current row's first or last peer in the <literal>ORDER BY</> ordering.
-    The <replaceable>value</> <literal>PRECEDING</> and
-    <replaceable>value</> <literal>FOLLOWING</> cases are currently only
-    allowed in <literal>ROWS</> mode.  They indicate that the frame starts
-    or ends with the row that many rows before or after the current row.
-    <replaceable>value</replaceable> must be an integer expression not
-    containing any variables, aggregate functions, or window functions.
-    The value must not be null or negative; but it can be zero, which
-    selects the current row itself.
    </para>
 
    <para>