bhyve.8: Improve the description and synopsis of -l

• Describe "-l help" separately for readability.
• List all the supported comX devices explicitly
• Use Cm instead of Ar for command modifiers (i.e., literal values a user can specify as an argument to the command).
• Explain where to get more information about the possible values of the conf argument.

MFC after: 2 weeks

(cherry picked from commit 4c08b978b276f8cfc25f72715e97898f629f7f89)