Summary:
Make kPageSize extern const size_t (used in draft https://github.com/facebook/rocksdb/issues/6427)
Make kLitteEndian constexpr bool
Clarify a couple of comments
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6443

Test Plan: make check, CI

Differential Revision: D20044558

Pulled By: pdillinger

fbshipit-source-id: e0c5cc13229c82726280dc0ddcba4078346b8418