powerd.8: Improve style & fix typos

• Sort options.
• Do not use macros (like .Ar) to specify width for Bl (macros within that string are not expanded).
• Use Cm instead of Ar for mode names.
• Fix some typos reported by mandoc.
• Move the documentation of the PID file from the -P flag description to the FILES section.

Approved by: bcr (mentor)
Differential Revision: https://reviews.freebsd.org/D23941