I know this answer is a little late, but I can't help myself, haha.
"notoriously knobbly legs" is a reference to skinny person. Typically skinny legs have less flesh on them so the bones are more visible. So skinny people are notorious or well known for their knobbly legs.
"by all female connoisseurs" in this context, is referring to the opinions of people (in this case more than likely men) that William's legs, in comparison to a beautiful woman's legs were pointedly condemned. A female connoisseur is a person that has an appreciation of the finer subtleties of a woman, much as a wine connoisseur has of wine.
"were magnificent shanks when it came to passage and bearing" refers back to the knobbly legs. The meaning here is his legs were strong and reliable and could take him many miles.
Hope this gives some more meaning and not confuse you :)