Move type/constructor signatures for alias types to top of docstrings

[abhro]

Checklist

• Appropriate tests were added
  None applicable
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• Any new documentation only uses public API

Additional context

Moves the type/constructor signature of Alias Specifier subtypes to the top of their respective docstrings. This allows Documenter.jl and terminal/REPL help viewer to pick it up as a signature, and follows convention of docstring order for better readability.