add serializer set callback documentation
plug new revision
add comma madness and lowercasing W
feature #3701 [Serializer] add documentation for serializer callbacks (cordoval)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3701).

Discussion
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[Serializer] add documentation for serializer callbacks

related https://github.com/symfony/symfony/issues/10229

| Q             | A                   |
| ---           | ---                 |
| Doc fix?      | no                  |
| New docs?     | yes                 |
| Applies to    | 2.3+                |
| Fixed tickets | self :baby:      |
| License       | CC-ASA 3.0 Unported |

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Commits
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b865b40 add comma madness and lowercasing W
efe2029 plug new revision
eabdbd0 add serializer set callback documentation