Document `RecogniseClassical` (at least internally)

[fingolfin]

There is no documentation at all for RecogniseClassical right now. We should say at least:

• what are valid inputs ("classical groups in natural representation"?)
• what are the outputs
• point out existence of DisplayRecog, useful for debugging
  • alternatively, change RecogniseClassical to return an actual object which has a ViewObj method based on DisplayRecog

Perhaps we actually want a slightly higher-level wrapper around it and use that instead, which returns a much smaller record with only the salient info. (For example, entries like sq1 or needMeataxe seem mainly for internal use?)