improve help message in perfectboundcover.py

Jonathan Neuhauserrequested to merge
joneuhauser/extensions:perfect-bound-cover-description into master

Closes #265 (closed)

I tried to look up the technical terms to describe them further. Argparser doesn't seem to care about the newlines (tried in Windows cmd and git shell)

@Moini, does this adress the comment you left in d8750d09 (comment 419187195)?

Merge request reports