Add comments to note the name abiguity between the MySQL push-down

optimization and the WHERE-clause push-down optimization.

FossilOrigin-Name: 3d5fb1ec7a0440072d6e3b957903c85d0f32b8b07207a1ef22d1a69cf5e664d1

src/sqliteInt.h

@@ -1906,7 +1906,7 @@ struct sqlite3 {
 #define SQLITE_CursorHints    0x00000400 /* Add OP_CursorHint opcodes */
 #define SQLITE_Stat4          0x00000800 /* Use STAT4 data */
    /* TH3 expects this value  ^^^^^^^^^^ to be 0x0000800. Don't change it */
-#define SQLITE_PushDown       0x00001000 /* The push-down optimization */
+#define SQLITE_PushDown       0x00001000 /* WHERE-clause push-down opt */
 #define SQLITE_SimplifyJoin   0x00002000 /* Convert LEFT JOIN to JOIN */
 #define SQLITE_SkipScan       0x00004000 /* Skip-scans */
 #define SQLITE_PropagateConst 0x00008000 /* The constant propagation opt */
@@ -3580,7 +3580,7 @@ struct Select {
 #define SF_View          0x0200000 /* SELECT statement is a view */
 #define SF_NoopOrderBy   0x0400000 /* ORDER BY is ignored for this query */
 #define SF_UFSrcCheck    0x0800000 /* Check pSrc as required by UPDATE...FROM */
-#define SF_PushDown      0x1000000 /* SELECT has be modified by push-down opt */
+#define SF_PushDown      0x1000000 /* Modified by WHERE-clause push-down opt */
 #define SF_MultiPart     0x2000000 /* Has multiple incompatible PARTITIONs */
 #define SF_CopyCte       0x4000000 /* SELECT statement is a copy of a CTE */
 #define SF_OrderByReqd   0x8000000 /* The ORDER BY clause may not be omitted */