procctl.2: Editing pass

• Add some missing .Pp macros after the end of literal blocks and some lists to ensure there is a blank line before the following text.

• Use an indent of Ds for nested lists to reduce excessive indentation and make the bodies of the nested list items easier to read.

• Various and sundry rewordings and clarifications.

Reviewed by: kib, emaste
Differential Revision: https://reviews.freebsd.org/D47782

(cherry picked from commit 8277c790179304159c2e4dcb1d99552518d5be8e)