Multiline description support (fixes #287)

Supports more verbose descriptions in AnnotatedContextReader,
as well as internal notes which aren’t printed with the descriptions.

Rebased my original pull request (#355) against 3.0.

The pre-existing regex has since been moved to a constant (`self::DOCLINE_TRIMMER_REGEX`). I've added a couple of new regexes. Do you want me to change those to constants as well? I think it'll make the code less readable though.

One of the added regexes ("Remove block comment syntax") is nearly the same as `self::DOCLINE_TRIMMER_REGEX`, but serves a slightly different purpose: Multiline checks, and retains whitespace apart from the first occurrence, in order to allow for visible indents in descriptions (e.g. for lists).