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

@@ -779,8 +779,8 @@ 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

@@ -1802,8 +1802,8 @@ FROM generate_series(1,10) AS s(i);
     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