libcasper(3): Correct some warnings found by mandoc

• inserting missing end of block: Sh breaks Bl
• moving content out of list: Pp
• missing comma before name: Nm cap_*
• comma in function argument: cap_*
• skipping paragraph macro: Pp after Sh
• sections out of conventional order: Sh AUTHORS

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D31144

(cherry picked from commit 3251ad29f4295ad73668a46727c5bbf7bbac0cf7)