Improve service-http-resp-content-type assertion

[mmccool]

• The use of "specifically" is a little confusing. What we want to say is to allow either application/json or application/td+json, but that td+json is more specific (and implies/includes application/json).
• We also need to do the corresponding thing in CoAP, simply allowing two numeric content codes for either application/json or application/td+json

[farshidtz]

This wording currently appears in three places:

wot-discovery/index.html

Lines 1052 to 1054 in 2356eb5

	<span class="rfc2119-assertion" id="exploration-server-http-resp-content-type">
	A successful response with JSON serialization MUST contain `application/json`
	or more specifically, `application/td+json` in the Content-Type header.</span>

wot-discovery/index.html

Lines 1622 to 1624 in 898f607

	<span class="rfc2119-assertion" id="tdd-things-retrieve-resp-content-type">
	A successful response with JSON serialization MUST contain `application/json`
	or more specifically, `application/td+json` in the Content-Type header.</span>

wot-discovery/index.html

Lines 1851 to 1853 in 2356eb5

	<span class="rfc2119-assertion" id="tdd-things-list-resp-content-type">
	A successful response with JSON serialization MUST contain `application/json`
	or more specifically, `application/ld+json` in the Content-Type header.</span>