Clarify options for resolving template variables

[handrews]

Addresses #288

If a variable can be resolved from either the instance or from
client data, the instance data may be used as a default when
asking for client data.

Also clarify how "hrefSchema" can handle a server-computed
URI path with client-supplied data string, as in traditional
HTML "get" forms. This should address @awwright's
#288 (comment)

[jdesrosiers]

I might be missing something, but I don't see how this changes anything. That dependency is still there. If anything this just seems to acknowledge that the dependency exists.

[handrews]

Given the state of things in #288, I am inclined to discard this unless @awwright or someone else thinks it is useful independent of the concerns in that issue.

[handrews]

I've decided that this is probably just confusing, and given that we don't have consensus on this as a desired interpretation it's better to leave it open and see what sort of feedback we get. In particular, we already have several discussions going around default values, and hopefully one of those will result in more philosophically consistent solutions.