Apply some upstream updates for 5.1 release note
Apply some upstream updates for 5.1 release note

以下のコミットを反映：
- https://github.com/rails/rails/commit/67550f16129a293a4bff76f4311b6eb552361aa1
- https://github.com/rails/rails/commit/ee7fb923d484f1dfe19e10210e7bde0a753ff2f5
- https://github.com/rails/rails/commit/d77823146afd8f770e3fee55423bf66630cd8459
- https://github.com/rails/rails/commit/027e492b6f873c58b1a531925f3a4dc6726ecc26

なお、以下のコミットは5.0のリリースノートとの整合を取る必要があるのでここでは反映していません
https://github.com/rails/rails/commit/df6aef2518884f65871f719125a972790518fd17
yasslab/railsguides.jp,yasslab/railsguides.jp,yasslab/railsguides.jp,yasslab/railsguides.jp