Clarify B/C Promise for parameter names on Attributes

[mbabker]

Presently, parameter names are not covered under the B/C promise. However, it is common practice to use named arguments with attributes, and this is even used in examples for the routing and validation docs.

Given that the majority of Symfony's attributes are built on top of pre-existing annotations classes that implicitly use named arguments (as they internally use associative array structures) and that changing these names would be a potential B/C break, could I ask that consideration be given to making the argument names for Attribute constructors a part of the B/C promise?

[nicolas-grekas]

To me, constructor arguments on attributes should be documented as being covered by the BC promise.

Up for a PR?

[mbabker]

See #15770