ShellPkg: Fix dynamic command help

- Use Shell Help print function that understands Shell Man Page format.
- Move Dynamic help print above the catchall case otherwise the help is
  printed multiple times in case of man page format.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Harry Liebel <Harry.Liebel@arm.com>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
Reviewed-by: Jaben Carsey <Jaben.Carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16053 6f19259b-4bc3-4df7-8a09-765794883524