Type correction in Section 5.4, and some editorial fixes

[k-toumura]

As pointed out in issue #120, 'resource type (rt)' is now used to register TDs to CoRE-RD.

And also this PR contains following editorial fixes:

• update CoRE-RD and JSONPath information in localBiblio.
• add rfc2119-assertion class for assertions in Section 5.
• use a term 'Thing Description Directory' instead of 'Directory Service' in Section 5.
• add 'IANA Considerations' section (for future registration for Resource Type Link Target Attribute, etc.)

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