Expand allomorph glossary

- Explain more what "two values" means
- Mention that Setties and Baggies care about object identity

doc/Language/glossary.pod6

@@ -72,7 +72,14 @@ Also see L<#Adverb> and L<#Colon Pair and Colon List>.
 X<|Allomorph>
 
 A type that has two related values which may be used depending on the
-context. The I<allomorph> types L<IntStr|/type/IntStr>, L<NumStr|/type/NumStr>,
+context. For example L<IntStr|/type/IntStr> allomorph C<is> both 
+an L<Int|/type/Int> and a L<Str|/type/Str>, so it will be accepted by
+anything that expects an C<Int>, a C<Str>, or an C<IntStr>. Keep in mind
+that certain constructs, such as L<sets, bags, and mixes|/language/setbagmix>
+care about object identity, and so will not accept an allomorph as equivalent
+of its components alone.
+
+The I<allomorph> types L<IntStr|/type/IntStr>, L<NumStr|/type/NumStr>,
 L<RatStr|/type/RatStr> and L<ComplexStr|/type/ComplexStr> may be created
 as a result of parsing a quoted string: