Change vague message for sort_constructors_first

[Levi-Lesches]

Describe the issue
Currently, the lint message is Invalid location for a constructor declaration. That doesn't exactly tell you what to change or how to fix the problem.

To Reproduce

class A {
  final int b;
  A(this.b);  // INFO | LINT Invalid location for a constructor declaration.
}

Expected behavior
Something clearer, like what's currently on the page for the lint:

Sort constructor declarations before other members.

Additional context
https://dart.dev/tools/linter-rules/sort_constructors_first

[pq]

/fyi @bwilkerson