Add class comment for Block

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/5832

Differential Revision: D17550773

Pulled By: ltamasi

fbshipit-source-id: 66972bb008516e55b6fbba58ddd10234346d5d11

table/block_based/block.h

@@ -135,6 +135,18 @@ class BlockReadAmpBitmap {
   uint32_t rnd_;
 };
 
+// This Block class is not for any old block: it is designed to hold only
+// uncompressed blocks containing sorted key-value pairs. It is thus
+// suitable for storing uncompressed data blocks, index blocks (including
+// partitions), range deletion blocks, properties blocks, metaindex blocks,
+// as well as the top level of the partitioned filter structure (which is
+// actually an index of the filter partitions). It is NOT suitable for
+// compressed blocks in general, filter blocks/partitions, or compression
+// dictionaries (since the latter do not contain sorted key-value pairs).
+// Use BlockContents directly for those.
+//
+// See https://github.com/facebook/rocksdb/wiki/Rocksdb-BlockBasedTable-Format
+// for details of the format and the various block types.
 class Block {
  public:
   // Initialize the block with the specified contents.