Revert "Merge pull request #16622 from matthewd/default-debug"

This reverts commit 2602a49a8600ab52f807599bbd5b1f9c0be4214f, reversing
changes made to 5d7c1057684c377bc2801c8851e99ff11ab23530.

The explicit default was introduced in 21f6d72, so apps created with Rails < 4
have the commented out version, which means that this change would break those
apps.
Deprecate different default for `log_level` in production

This is a more conservative approach to 2602a49. Also changed the comment to be
more inline with everything else in the file (describing what the config value
is doing and why). People should just read the docs for alternatives.