Mention help about confusing fields / methods

[nikomatsakis]

@Nashensas88 commented:

If we do leave it in, is this the right place to mention that the existing help span should be extended? There's one in place now that will recognize a field is a closure and rewrites your code with the parenthesis as a suggestion.

This is a good point! Specifically, we should mention in the "How do we teach this?" section that we would want to extend the existing text about confusing field vs method access in structs and other types to apply to traits.

[nikomatsakis]

This seems like a no-brainer. PRs would be welcome on this point!