Clarify wording around fragid syntaxes

Only schema+json supports two syntaxes, so only it needs a rule
for disambiguating them.  schema-instance+json only supports one.

Author: handrews

jsonschema-core.xml

@@ -332,9 +332,9 @@
             <t>
                 The use of JSON Pointers as URI fragment identifiers is described in
                 <xref target="RFC6901">RFC 6901</xref>.
-                For "application/schema+json", fragment identifiers matching the JSON Pointer
-                syntax, including the empty string, MUST be interpreted as JSON Pointer fragment
-                identifiers.
+                For "application/schema+json", which supports two fragment identifier syntaxes,
+                fragment identifiers matching the JSON Pointer syntax, including the empty string,
+                MUST be interpreted as JSON Pointer fragment identifiers.
             </t>
             <t>
                 Per the W3C's