Deduplicate VerifyOption documentation

The VerifyOption enum is used a few times throughout the gc sources; in most of its uses the method description documents the possible enum values, like ~11 times:

  // vo == UsePrevMarking -> use "prev" marking information,
  // vo == UseNextMarking -> use "next" marking information
  // vo == UseMarkWord -> use the mark word in the object header

  // ... lengthy description of these values

Move the documentation of the VerifyOption values to the enum definition, and remove the duplicates.