Minor comment changes. Add ALWAYS() macros on some unreachable branches

in the xStep() methods of built-in window functions.

FossilOrigin-Name: f2057542cf6860badb4ae6b1a0df94a78d5b2661dabf16f5705b3f8236521bea

src/sqliteInt.h

@@ -3515,7 +3515,7 @@ struct Window {
   Window *pNextWin;       /* Next window function belonging to this SELECT */
   Expr *pFilter;          /* The FILTER expression */
   FuncDef *pFunc;         /* The function */
-  int iEphCsr;            /* Temp table used by this window */
+  int iEphCsr;            /* Partition buffer or Peer buffer */
   int regAccum;
   int regResult;
   int csrApp;             /* Function cursor (used by min/max) */