Fix incorrect markup in documentation of window frame clauses.

You're required to write either RANGE or ROWS to start a frame clause,
but the documentation incorrectly implied this is optional.  Noted by
David Johnston.

doc/src/sgml/ref/select.sgml

@@ -651,8 +651,8 @@ WINDOW <replaceable class="parameter">window_name</replaceable> AS ( <replaceabl
     The <replaceable class="parameter">frame_clause</> can be one of
 
 <synopsis>
-[ RANGE | ROWS ] <replaceable>frame_start</>
-[ RANGE | ROWS ] BETWEEN <replaceable>frame_start</> AND <replaceable>frame_end</>
+{ RANGE | ROWS } <replaceable>frame_start</>
+{ RANGE | ROWS } BETWEEN <replaceable>frame_start</> AND <replaceable>frame_end</>
 </synopsis>
 
     where <replaceable>frame_start</> and <replaceable>frame_end</> can be

doc/src/sgml/syntax.sgml

@@ -1725,8 +1725,8 @@ SELECT string_agg(a ORDER BY a, ',') FROM table;  -- incorrect
     and the optional <replaceable class="parameter">frame_clause</replaceable>
     can be one of
 <synopsis>
-[ RANGE | ROWS ] <replaceable>frame_start</>
-[ RANGE | ROWS ] BETWEEN <replaceable>frame_start</> AND <replaceable>frame_end</>
+{ RANGE | ROWS } <replaceable>frame_start</>
+{ RANGE | ROWS } BETWEEN <replaceable>frame_start</> AND <replaceable>frame_end</>
 </synopsis>
     where <replaceable>frame_start</> and <replaceable>frame_end</> can be
     one of