@max_depth: accepted on type aliases but unused there, and its value isn't checked #199

Found while documenting bounds (docs#142, core#98):

• Ignored on aliases: @max_depth is accepted on type aliases, but only structs are checked or filled from it, so on an alias it silently does nothing. Either apply it to aliases or refuse it there.
• Value not checked: the annotation's value isn't required to be at least 1, while the bounds.depth.max setting is. @max_depth = 0 should be refused with the same range as the setting.

The docs currently document @max_depth on structs only.