Fixup Device Classes

[mmccool]

Resolves #598

• fix reference to RFC7228
• use "gateway/hub" for Class 4
• Move section to more appropriate location (it's not really a building block; perhaps move under Terminology?)
• General grammar and positioning cleanup (e.g. sentence in which RFC is cited is confusing)

Note: "hub" is only mentioned in this section now and no-where else. So the definition of hub also needs to be addressed, but this should be dealt with in a different PR that addresses issue #581

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[mmccool]

When revising the text I found I was not entirely happy with the device categories, so added some comments for some other possible names and also an ednote discussing some other options. We could either merge this as is, or discuss (and possibly revise) the names and add a separate section on security (rather than including them in the names) and then remove the ednote.