Add header comment for sqlite3_vtab_collation().

FossilOrigin-Name: d238694ca445ccb4eeb3e3269a5f872f998f795945d0f9dd95c11d0e42d4d538
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index 22892c9..259c020 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -1399,7 +1399,7 @@
   VtabCtx *pVtabCtx;            /* Context for active vtab connect/create */
   VTable **aVTrans;             /* Virtual tables with open transactions */
   VTable *pDisconnect;    /* Disconnect these in next sqlite3_prepare() */
-  void *pVtabWC;                /* For sqlite3_vtab_collation() */
+  void *pBestIndexCtx;          /* For sqlite3_vtab_collation() */
 #endif
   Hash aFunc;                   /* Hash table of connection functions */
   Hash aCollSeq;                /* All collating sequences */