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

optimization and the WHERE-clause push-down optimization.

FossilOrigin-Name: 3d5fb1ec7a0440072d6e3b957903c85d0f32b8b07207a1ef22d1a69cf5e664d1

src/wherecode.c

@@ -2474,6 +2474,12 @@ Bitmask sqlite3WhereCodeOneLoopStart(
   ** iLoop==3: Code all remaining expressions.
   **
   ** An effort is made to skip unnecessary iterations of the loop.
+  **
+  ** This optimization of causing simple query restrictions to occur before
+  ** more complex one is call the "push-down" optimization in MySQL.  Here
+  ** in SQLite, the name is "MySQL push-down", since there is also another
+  ** totally unrelated optimization called "WHERE-clause push-down".
+  ** Sometimes the qualifier is omitted, resulting in an ambiguity, so beware.
   */
   iLoop = (pIdx ? 1 : 2);
   do{