Some doc improvements

[cgay]

Copied from dylan-lang/opendylan#576 ...

Some good suggestions from brucem...

• docs for %{pid} says it isn't implemented yet.
• docs for the date stuff should link to format-date's docs.
• would be nice to have both thread ID and name as format options.
• How do these options compare to log4j or logback?
• if i want to create a custom log formatter (maybe not text or more structured), what would I override to do that?
• The things that say they shouldn't be called unless you're extending logging ... why not export them from a different module?
• The roll keyword could be more verbose to be more clear...