Add more documentation to declaring conditionals.

[deech]

[sol]

@deech First thing first, hanks for taking the time to open a PR.

As it currently stands, it's more like "this surprised me, and this is the workaround". But those things are not really surprising, they result from how YAML works.

I'm not suggesting we should not address this. But, can we address this in a way so that the user gets an understanding of the underlying mechanics (YAML, in this case)?

[deech]

Hi, Thanks for hpack! I loveit.

Having read your comments I agree that this is a YAML and not an hpack issue. I'm obviously not as familiar with YAML but as a Cabal user it did trip me up for a few hours. How do you feel about amending the PR to acknowledge that these "quirks" are consistent with underlying markup format but might be confusing to newcomers?

[sol]

@deech You are welcome! Please give it a try!

[sol]

@deech hpack now warns on duplicate fields. So I think the main source of error has been removed! From what I understand, what is left is still that ! needs quoting. I'll cherry-pick the corresponding parts from you PR when I have time to do so.