Editorial: Clarify that the Burmese calendar is not described in the spec

The Burmese calendar is used as an example here of era year 0, but is not
one of the CLDR calendars. Say "not currently available in CLDR" and not
"not supported" because this text should be mirrored in Temporal where it
is not required to be supported anyway.

Author: ptomato

spec/calendar.html

@@ -73,7 +73,7 @@ <h1>Calendar Date Records</h1>
             <td>
               The ordinal position of the date's year within its era, or *undefined* for calendars that do not have eras.
               <emu-note>
-                Era years are 1-indexed for many calendars, but not all (e.g., the eras of the Burmese calendar each start with a year 0). Years can also advance opposite the flow of time (as for BCE years in the Gregorian calendar).
+                Era years are 1-indexed for many calendars, but not all (e.g., the eras of the Burmese calendar, not currently available in CLDR, each start with a year 0). Years can also advance opposite the flow of time (as for BCE years in the Gregorian calendar).
               </emu-note>
             </td>
           </tr>