Enhanced documentation and minor code tweaks in preparation for hardening

the sqlite3_initialize/shutdown interfaces against initialization failures.

FossilOrigin-Name: 98c49e6135ae6268a80de88f8b0284f88ef32e1d

src/malloc.c

@@ -43,7 +43,9 @@ void sqlite3_soft_heap_limit(int n){
   }else{
     iLimit = n;
   }
+#ifndef SQLITE_OMIT_AUTOINIT
   sqlite3_initialize();
+#endif
   if( iLimit>0 ){
     sqlite3MemoryAlarm(softHeapLimitEnforcer, 0, iLimit);
   }else{