Add a version number to the sqlite3_pcache_methods2 object. Other PCACHE2

documentation improvements.

FossilOrigin-Name: 9f839ac05a9f3cfe587d2ccdccd50dac41baedbe

src/test_init.c

@@ -160,7 +160,7 @@ static void installInitWrappers(void){
     wrMutexLeave, wrMutexHeld,  wrMutexNotheld
   };
   sqlite3_pcache_methods2 pcachemethods = {
-    0,
+    1, 0,
     wrPCacheInit,      wrPCacheShutdown,  wrPCacheCreate, 
     wrPCacheCachesize, wrPCachePagecount, wrPCacheFetch,
     wrPCacheUnpin,     wrPCacheRekey,     wrPCacheTruncate,