Issue #12658 Add a deprecation note about Translator's setFallbackLocale...

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12658
| License       | MIT
| Doc PR        | 
Can you move the whole expression on one line? Thanks.