Fix registry.dtd to not use alternatives in function, formatSignature…

…, matchSignature

A spin-off from unicode-org#420, in which @mihnita noticed that `registry.dtd` uses alternatives in a wrong way. For example, `<!ELEMENT formatSignature (input?|option*)>` means that `formatSignature` is allowed to have as children either: at most one `input` OR any number of `options`, but not both.

Instead, @mihnita suggested using sequences: `<!ELEMENT formatSignature (input?,option*)>`, which this PR does. Note that sequences require the children to appear in a specific order, which isn't something that's useful to us. However, I'm not aware of any way of lifting this requirement that also allows to enforce at most on `input` or exactly one `description`.

spec/registry.dtd

@@ -1,6 +1,6 @@
-<!ELEMENT registry (function*|pattern*)>
+<!ELEMENT registry (function|pattern)*>
 
-<!ELEMENT function (description|(formatSignature|matchSignature)+)>
+<!ELEMENT function (description,(formatSignature|matchSignature)+)>
 <!ATTLIST function name NMTOKEN #REQUIRED>
 
 <!ELEMENT description (#PCDATA)>
@@ -9,11 +9,11 @@
 <!ATTLIST pattern id ID #REQUIRED>
 <!ATTLIST pattern regex CDATA #REQUIRED>
 
-<!ELEMENT formatSignature (input?|option*)>
+<!ELEMENT formatSignature (input?,option*)>
 <!ATTLIST formatSignature position (open|close|standalone) "standalone">
 <!ATTLIST formatSignature locales NMTOKENS #IMPLIED>
 
-<!ELEMENT matchSignature (input?|option*|match*)>
+<!ELEMENT matchSignature (input?,option*,match*)>
 <!ATTLIST matchSignature locales NMTOKENS #IMPLIED>
 
 <!ELEMENT input EMPTY>