doc: use terms like `logs`, `STDOUT`, `STDERR`

The general term `output` may be confusing, because it can mean other
items generated by a program. Where possible, use specific terminology
for log files.
log: improve terminology.
Merge pull request #797 from matthewrmshin/log

doc: use terms like `logs`, `STDOUT`, `STDERR`
Merge branch 'master' of github.com:metomi/rose into metomi-gh-pages
Merge branch 'master' of github.com:metomi/rose into gh-pages
Merge branch 'gh-pages' of github.com:metomi/rose into gh-pages