(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
(PUP-1473) Fix issue with UTF-8 in user comments

GH-1943 included a change to munge the comment property on the user
type so that it gets encoded as ASCII-8BIT. This was to allow the comment
property to handle UTF-8 values as existing values are read as ASCII-8BIT.

Originally the change called `force_encoding` to force the encoding to
ASCII-8BIT. This method does not actually attempt to encode the string, it
only changes the string's associated encoding. It was suggested during code
review that a call to `encode` should be used instead. However, calling
`encode` will fail if the string has UTF-8 characters that cannot be
represented in ASCII-8BIT.

As we desire the comment property to be treated as a squence of bytes in
this case, we should be calling `force_encoding` instead of `encode`.