rc.conf.5: Improve documentation of <name>_oomprotect

Apart from improving readability, this commit mentions that
<name>_oomprotect is ignored in a jail environment. Also, replace
${name}_cmd with the correct ${argument}_cmd and point the reader to
rc.subr(8).

MFC after: 1 week

(cherry picked from commit df1817a81e891c4850157e7f21f94b113d663bec)