CDS to DDL: clarify treatment of expressions

[stewsk]

No description provided.

[danjoa]

Why did we have to add this? → should make it more specific as stated generically like that it is also wrong. In particular: Expressions in @assert annotations are database-agnostic, which is done by the respective expression evaluator. Here: the CAP runtimes.
In general: expressions in annotations are rarely evaluated / executed by the compiler. So stating expressions are not agnostic from a compiler-only perspective is misleading.

[stewsk]

Why did we have to add this?

because the statement "CDS models are designed to be database-agnostic, " by itself is also misleading. With respect to expressions that makes the impression that one could write any expression and CAP ensures that it translated correctly to the respective database - like in ABAP. This is not true for the compiler, where all expressions are sent to the DB one-to-one, and even the runtimes only provide this service for certain expressions.

(reason for this addendum: there are cap/issues rooted in the expectation that one can just write any expression and that would work)