docs: reorg feature analysis TOC
docs: add Font.underline feature analysis

Also add MSO_TEXT_UNDERLINE_TYPE enumeration and docs.
acpt: add scenarios for Font.bold

* didn’t add them during original implementation of Font.bold
* improve wording of Font.size then clause
acpt: add scenarios for Font.italic getter

* refactor scenarios for Font.italic setter
acpt: add scenarios for Font.underline
txt: add Font.underline getter
test: refactor DescribeFont

* modernize fixtures for bold, italic, and size
* fix size to allow assignment of None to revert sz value to inherited
txt: add Font.underline setter