CLDR-17298 Fix problem with unit exponents (#4071)

docs/ldml/tr35-general.md

@@ -951,15 +951,17 @@ Some of the constraints reference data from the unitIdComponents in [Unit_Conver
     </ul></td></tr>
 
 <tr><td><a name='unit_constant' href='unit_constant'>unit_constant</a></td><td>:=</td>
-    <td>("1"[0-9]+ | [2-9][0-9]*)("e" ("1"[0-9]+ | [2-9][0-9]*))?
+    <td>[1-9][0-9]* ("e" [1-9][0-9]*)?
         <ul><li><em>Examples:</em>
             <ul><li>kilowatt-hour-per-100-kilometer</li>
                 <li>gallon-per-100-mile</li>
                 <li>per-200-pound</li>
                 <li>per-12</li>
             </ul></li>
-            <li><em>Note:</em> The number is an integer greater than one.</li>
+            <li><em>Constraint:</em> The numeric value of the unit constant must be an integer greater than one.</li>
+            <li><em>Note:</em> The normal interpretation of <code>e</code> is used, where 2e6 = 2×10⁶.</li>
             <li><em>Note:</em> The <code>e</code> notation is optional: per-100-kilometer and per-1e2-kilometer are equivalent unit_identifiers.</li>
+            <li><em>Note:</em> When constructing identifiers, exponents should be greater than 3 and multiples of 3, even though parsers must accept the wider range.</li>
         </ul></td></tr>
 
 <tr><td><a name='dimensionality_prefix' href='dimensionality_prefix'>dimensionality_prefix</a></td><td>:=</td>