Fixed some typos, use of RFC 2119 and links

[cyenyxe]

Some typos fixed, and added use of RFC 2119 in a couple of places. The link to the Interbase Interval Tests is now fixed, but it points to an archived file; it should be decided whether to keep it.

In addition to these small changes, I have some suggestions or questions from the terms and model index page:

• Receiving systems SHOULD NOT persist Ids from remote sources. Instead, Identifiers (below) SHOULD be used for communication between systems. -> MUST (NOT) instead of SHOULD (NOT)?
• Use of 'MUST' instead of 'SHALL' could remove the ambiguity that word has (can be a suggestion or a command)

[ahwagner]

Looks great, thanks!

[cyenyxe]

Fantastic! What do you think about the couple of questions raised in my first comment, should I address them or leave them be?

[ahwagner]

I'm not sure on bullet 1; there may have been a reason this was written as SHOULD NOT; perhaps there may be implementations that wish to for reasons we haven't considered? Anyway, I agree with you, but we should get feedback from @rrfreimuth @larrybabb and/or @reece first.

I agree with you on point 2, please replace any instances you find (though I'm not sure there are any?)