free-floating URLs not parsed well by documenter

[kris-brown]

@slwu89 points out that the docstring for image is broken. It is rendered as plain text "https://en.wikipedia.org/wiki/Image(categorytheory)#Second_definition" instead of a hyperlink to here

[epatters]

Thanks for the report.

• We should probably consider it bad style to put free-floating URLs in docstrings
• If we are going to do that, then the correct Markdown syntax is to put the URL in angle brackets: <https://...>