Update documentation about serializable keys in the execution context

[fmbenhassine]

The documentation section about the execution context should mention that non transient keys should be Serializable (otherwise a custom serializer should be used).

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Related-to: spring-projects/spring-framework#28432

[hwan33]

@fmbenhassine Can I work on this issue?

[fmbenhassine]

@hwan33 This was planned for the internal team but sure! Thank you for your offer to help!