Add suggested restrictions on the form of a term to avoid conflict with a URI scheme or a Compact IRI.

Fixes #429.

(cherry picked from commit 1069c14)

Author: gkellogg

spec/latest/json-ld/index.html

@@ -3157,7 +3157,16 @@ <h2>Terms</h2>
       to an <a>IRI</a> or a <a>blank node identifier</a>.</p>
 
     <p>A <a>term</a> MUST NOT equal any of the JSON-LD
-      <a>keyword</a>.</p>
+      <a>keywords</a>.</p>
+
+    <p class="changed">When used as the <a>prefix</a> in a <a>Compact IRI</a>, to avoid
+      the potential ambiguity of a <a>prefix</a> being confused with an IRI
+      scheme, <a>terms</a> SHOULD NOT come from the list of URI schemes as defined in
+      [[!IANA-URI-SCHEMES]]. Similarly, to avoid confusion between a
+      <a>Compact IRI</a> and a <a>term</a>, terms SHOULD NOT include a colon (<code>:</code>)
+      and SHOULD be restricted to the form of
+      <code><a data-cite="RFC3897#section-2.2">isegment-nz-nc</a></code>
+      as defined in [[!RFC3897]].</p>
 
     <p>To avoid forward-compatibility issues, a <a>term</a> SHOULD NOT start
       with an <code>@</code> character as future versions of JSON-LD may introduce