Document `ExtensionAwareEvaluationContextProvider` mechanism

[mp911de]

As raised in spring-projects/spring-data-relational#1719, we should document the general SpEL mechanism. We also have a ticket to extend the mechanism via #2369 so in that context, it would make sense to have documentation fragment that explains the default scope, limits and extension mechanism for our SpEL support.

[schauder]

While looking into this I found https://docs.spring.io/spring-data/commons/reference/value-expressions.html#_extending_the_evaluation_context
which seems fairly complete to me.

[mp911de]

This was introduced with Value Expressions, seems this ticket was superseded by #2369