Additional edits for clarity and consistency

[aphillips]

As part of a final read-through before we go wider with review, I made edits for consistency and clarity. These include:

• made links to valid and well-formed include the word language tag[s] as needed
• corrected capitalization of I18N -> i18n (our house style) and removed standalone use in the introduction
• moved one requirement down so that it will make more sense
• clarified RFC4646's relationship to BCP47
• added a sentence to clarify that 4646's grammar is stable and unlikely to change
• clarified that terminology is taken from BCP47 (not invented here)

These changes are all rather editorial, so I'm going to merge them directly.

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