rc: add a manual entry for ${name}_setup

${name}_prepend is suboptimal as it is prepended to the actual
command being run. Therefore the term "commandS to prepend"
is misleading and no clear separation takes place for setup tasks
that may be required like changing a config file permission or
generating a configuration file prior to service start.

The other reason is that {argument}_precmd is a service-side
variable and cannot be repurposed from the user-side.

Reviewed by: pauamma
Differential Revision: https://reviews.freebsd.org/D36210