Clarify io_activity (#11427)

Summary:
Document ReadOptions::io_activity as internal-use-only. And to keep kUnknown as last (and why).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11427

Test Plan: comments only

Reviewed By: hx235

Differential Revision: D45576986

Pulled By: pdillinger

fbshipit-source-id: aae15aa22ea91370c2b7366154e45d4b91a79ad2

include/rocksdb/env.h

@@ -441,7 +441,7 @@ class Env : public Customizable {
     kFlush = 0,
     kCompaction = 1,
     kDBOpen = 2,
-    kUnknown,
+    kUnknown,  // Keep last for easy array of non-unknowns
   };
 
   // Arrange to run "(*function)(arg)" once in a background thread, in

include/rocksdb/options.h

@@ -1696,6 +1696,7 @@ struct ReadOptions {
   // Default: true
   bool optimize_multiget_for_io;
 
+  // ** For RocksDB internal use only **
   Env::IOActivity io_activity;
 
   ReadOptions();