Fix comments and typos

Summary: Correct some comments and typos in RocksDB.

Test Plan: Inspection

Reviewers: sdong, igor

Reviewed By: igor

Differential Revision: https://reviews.facebook.net/D23133

db/column_family.h

@@ -133,7 +133,7 @@ class ColumnFamilyData {
   void Ref() { ++refs_; }
   // will just decrease reference count to 0, but will not delete it. returns
   // true if the ref count was decreased to zero. in that case, it can be
-  // deleted by the caller immediatelly, or later, by calling
+  // deleted by the caller immediately, or later, by calling
   // FreeDeadColumnFamilies()
   bool Unref() {
     assert(refs_ > 0);

db/compaction_picker.cc

@@ -42,13 +42,13 @@ CompressionType GetCompressionType(const Options& options, int level,
     return kNoCompression;
   }
   // If the use has specified a different compression level for each level,
-  // then pick the compresison for that level.
+  // then pick the compression for that level.
   if (!options.compression_per_level.empty()) {
     const int n = options.compression_per_level.size() - 1;
     // It is possible for level_ to be -1; in that case, we use level
     // 0's compression.  This occurs mostly in backwards compatibility
     // situations when the builder doesn't know what level the file
-    // belongs to.  Likewise, if level_ is beyond the end of the
+    // belongs to.  Likewise, if level is beyond the end of the
     // specified compression levels, use the last value.
     return options.compression_per_level[std::max(0, std::min(level, n))];
   } else {

include/rocksdb/db.h

@@ -123,7 +123,7 @@ class DB {
 
   // Open DB with column families.
   // db_options specify database specific options
-  // column_families is the vector of all column families in the databse,
+  // column_families is the vector of all column families in the database,
   // containing column family name and options. You need to open ALL column
   // families in the database. To get the list of column families, you can use
   // ListColumnFamilies(). Also, you can open only a subset of column families

include/rocksdb/options.h

@@ -224,17 +224,12 @@ struct ColumnFamilyOptions {
   CompressionType compression;
 
   // Different levels can have different compression policies. There
-  // are cases where most lower levels would like to quick compression
+  // are cases where most lower levels would like to use quick compression
-  // algorithm while the higher levels (which have more data) use
+  // algorithms while the higher levels (which have more data) use
   // compression algorithms that have better compression but could
-  // be slower. This array, if non nullptr, should have an entry for
+  // be slower. This array, if non-empty, should have an entry for
-  // each level of the database. This array, if non nullptr, overides the
+  // each level of the database; these override the value specified in
-  // value specified in the previous field 'compression'. The caller is
+  // the previous field 'compression'.
-  // reponsible for allocating memory and initializing the values in it
-  // before invoking Open(). The caller is responsible for freeing this
-  // array and it could be freed anytime after the return from Open().
-  // This could have been a std::vector but that makes the equivalent
-  // java/C api hard to construct.
   std::vector<CompressionType> compression_per_level;
 
   // different options for compression algorithms