Mention CacheableSupportsMethodInterface in "Create your own Normalizer"

[Steveb-p]

Introduced in 4.1, blog post: https://symfony.com/blog/new-in-symfony-4-1-faster-serializer

It is mentioned, but only in "performance" subpart of docs: https://symfony.com/doc/current/components/serializer.html#performance

General serializer docs regarding Normalizers: https://symfony.com/doc/current/components/serializer.html#normalizers

Creating your own Normalizer misses this completely: https://symfony.com/doc/current/serializer/custom_normalizer.html