Rework pkg-set.8

[pauamma]

• Reorganize synopsis to pair short and long options

• Mark exclusive options as such

• Add missing --vital synonym for -v

• Reorder sections to pacify mandoc -T lint

• Copyedit and fix markup throughout

[pauamma]

Planning to go over all manual pages. Will mark as mergeable when done.

[0mp]

Hey Pau Amma, thanks for working on the manual pages.

I’d like to suggest dropping the long options from the synopsis entirely for readability. See #2027. This way it would also be less editing work for you.

[vstakhov]

I'm not even sure if there is any useful outcome of just listing all possible options and arguments (especially optional ones) in the same place. Short options only are totally useless as they provide no information about what they do. Long options are, indeed, messy... So a format like pkg set <mandatory_arguments_and_options> [OPTIONS] look more sane: we define the mandatory long/short options/arguments and hide everything else in that [OPTIONS] block, describing all those options later.