Update grammar for property generators to match algorithms

This addresses #142.

Author: lanthaler

spec/latest/json-ld-syntax/index.html

@@ -3004,7 +3004,10 @@ <h2>Context Definition</h2>
   include the <code>@id</code> key.</p>
 
 <p>If the <tref>expanded term definition</tref> contains the <code>@id</code> <tref>keyword</tref>,
-  its value MUST be <code>null</code>, an <tref>IRI</tref>, a <tref>compact IRI</tref>, a <tref>term</tref> defined in the defining <tref>context definition</tref> or the <tref>active context</tref>, or an <tref>array</tref> composed of any of the previous allowed values.</p>
+  its value MUST be <code>null</code>, an <tref>absolute IRI</tref>, a <tref>blank node identifier</tref>,
+  a <tref>compact IRI</tref>, a <tref>term</tref> defined in the defining <tref>context definition</tref>
+  or the <tref>active context</tref>, or an <tref>array</tref> composed of any of the previous allowed values except
+  <tref>null</tref>.</p>
 <p>If the <tref>expanded term definition</tref> contains the <code>@type</code> <tref>keyword</tref>,
   its value MUST be an <tref>absolute IRI</tref>, a <tref>compact IRI</tref>, a <tref>term</tref> defined in the defining <tref>context definition</tref> or the <tref>active context</tref>, or the <code>@id</code> <tref>keyword</tref>.</p>
 <p>If the <tref>expanded term definition</tref> contains the <code>@language</code> <tref>keyword</tref>,