(docs) Update description for Exec type's logoutput parameter

The logoutput parameter can't accept log levels as legal values. This has been
wrong since 2008 (commit bb8051bc406d1da67db8212e852bb36d1368e953) and it sounds
like it was practically wrong long before that.
Merge pull request #2343 from nfagerlund/docs_exec_logoutput

(docs) Update description for Exec type's logoutput parameter