Add description to Schema

At a glance

• Identifier: #466
• Stage: RFC3: Accepted
• Champion: @IvanGoncharov
• PR: Add description to Schema

Timeline

• Spec PR created on 2018-06-13 by IvanGoncharov
• 2 commits pushed on 2018-06-13:
  • Move description section by @IvanGoncharov
  • Add description to Schema by @IvanGoncharov

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Fist comment is just put Description section before Schema section. Changes to review: bd5cdd8

This PR adds description to schema both in SDL and introspection.