Merge the HTTP guidance into the "consult other specs" section.

[jyasskin]

@ylafon, the removed "See" bit comes from your comment in #379, so feel free to object.

I see we have some other proposals to add web-specific HTTP guidance. This PR doesn't preclude adding back a less-redundant section on this space. :)

Fixes #545.

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[martinthomson]

I'm OK with this, on the basis that we'll likely want to do something about #512 at some point.

[jyasskin]

I wonder if we should create a policy of preserving all section ids...