fix some trifling points

doc/index.md

@@ -345,7 +345,7 @@ non-NULL, it is used to cache frequently used uncompressed block contents.
 #include "leveldb/cache.h"
 
 leveldb::Options options;
-options.block_cache = leveldb::NewLRUCache(100 * 1048576);  // 100MB cache
+options.block_cache = leveldb::NewLRUCache(100 * 1048576);  // 100M cache capacity
 leveldb::DB* db;
 leveldb::DB::Open(options, name, &db);
 ... use the db ...
@@ -369,6 +369,7 @@ leveldb::Iterator* it = db->NewIterator(options);
 for (it->SeekToFirst(); it->Valid(); it->Next()) {
   ...
 }
+delete it;
 ```
 
 ### Key Layout
@@ -424,21 +425,21 @@ spaces. For example:
 ```c++
 class CustomFilterPolicy : public leveldb::FilterPolicy {
  private:
-  FilterPolicy* builtin_policy_;
+  leveldb::FilterPolicy* builtin_policy_;
 
  public:
-  CustomFilterPolicy() : builtin_policy_(NewBloomFilterPolicy(10)) {}
+  CustomFilterPolicy() : builtin_policy_(leveldb::NewBloomFilterPolicy(10)) {}
   ~CustomFilterPolicy() { delete builtin_policy_; }
 
   const char* Name() const { return "IgnoreTrailingSpacesFilter"; }
 
-  void CreateFilter(const Slice* keys, int n, std::string* dst) const {
+  void CreateFilter(const leveldb::Slice* keys, int n, std::string* dst) const {
     // Use builtin bloom filter code after removing trailing spaces
-    std::vector<Slice> trimmed(n);
+    std::vector<leveldb::Slice> trimmed(n);
     for (int i = 0; i < n; i++) {
       trimmed[i] = RemoveTrailingSpaces(keys[i]);
     }
-    return builtin_policy_->CreateFilter(trimmed.data(), n, dst);
+    builtin_policy_->CreateFilter(trimmed.data(), n, dst);
   }
 };
 ```