bpdoc preformatted text improvments

1. Apply preformatted text (<pre>...</pre>) handling logic to module
type text too. It used to be applied to property texts only.
2. Improve <pre> handling logic itself for better readability.
- Insert a blank line before <pre>.
- Prevent from ending <pre> blocks prematurely by checking if an
unindented line isn't just a blank line between indented lines.

Change-Id: Id40fa668d4c6781caf7ed140b2f40784cdeb8c35