Code comment for Version Edit (#5829)

Summary:
Added comment for Version Edit.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5829

Test Plan: Run existing tests

Differential Revision: D17486229

Pulled By: vjnadimpalli

fbshipit-source-id: b4b31104fadd667356b64bd2dc409b3376ee46ca

db/version_edit.h

@@ -191,6 +191,10 @@ struct LevelFilesBrief {
   }
 };
 
+// The state of a DB at any given time is referred to as a Version.
+// Any modification to the Version is considered a Version Edit. A Version is
+// constructed by joining a sequence of Version Edits. Version Edits are written
+// to the MANIFEST file.
 class VersionEdit {
  public:
   VersionEdit() { Clear(); }