[fr][ja] Follow #1853
Correct isUnique example with multiple fields

Closes https://github.com/cakephp/cakephp/issues/4869
Merge pull request #1863 from cakephp/isunique-multiple-fields-example

Correct isUnique example with multiple fields
Merge pull request #1854 from blackpuppy/zh

[zh_CN] done /zh/models/associations-linking-models-together.rst
[fr] follows #1863
[zh_CN] delete original text