Update db.rst
Note for adding related entities in models
Updated withing =]
[en] Fixed a mistake
[ja] Translated some headings
Merge pull request #273 from inouet/patch-7

[en] Fixed a mistake,  [ja] Translated some headings
Fixed mistake and translated one sentence
Merge pull request #274 from JimmDiGrizli/patch-4

Fixed mistake and translated one sentence
Update whats-next.rst
Update whats-next.rst
fixing a type, and finishing a sentance
Merge pull request #251 from Chameleon-m/patch-22

Update db.rst
Update db.rst
Merge pull request #275 from ritter/patch-1

fixing a type, and finishing a sentance
Merge pull request #262 from WooDzu/patch-1

Note for adding related entities in models