[spec/enum] Tweak docs (#3656)

Move EnumMember above EnumMemberAttributes, as it is mentioned first.
Split anonymous enums into separate grammar block.
Add subheading for single member syntax.
Explain that all enum members are manifest constants.
Fix Issue 23571 - Discussion of manifest constants in enum documentation
is confusing at best.

Author: ntrel

spec/enum.dd

@@ -22,6 +22,10 @@ $(GNAME EnumMembers):
     $(GLINK EnumMember) $(D ,)
     $(GLINK EnumMember) $(D ,) $(GSELF EnumMembers)
 
+$(GNAME EnumMember):
+    $(GLINK EnumMemberAttributes)$(OPT) $(GLINK_LEX Identifier)
+    $(GLINK EnumMemberAttributes)$(OPT) $(GLINK_LEX Identifier) $(D =) $(ASSIGNEXPRESSION)
+
 $(GNAME EnumMemberAttributes):
     $(GLINK EnumMemberAttribute)
     $(GLINK EnumMemberAttribute) $(GSELF EnumMemberAttributes)
@@ -30,11 +34,8 @@ $(GNAME EnumMemberAttribute):
     $(GLINK2 attribute, DeprecatedAttribute)
     $(GLINK2 attribute, UserDefinedAttribute)
     $(D @)$(LINK2 attribute.html#disable, $(D disable))
-
-$(GNAME EnumMember):
-    $(GLINK EnumMemberAttributes)$(OPT) $(GLINK_LEX Identifier)
-    $(GLINK EnumMemberAttributes)$(OPT) $(GLINK_LEX Identifier) $(D =) $(ASSIGNEXPRESSION)
-
+)
+$(GRAMMAR
 $(GNAME AnonymousEnumDeclaration):
     $(D enum) $(D :) $(GLINK EnumBaseType) $(D {) $(GLINK EnumMembers) $(D })
     $(D enum) $(D {) $(GLINK AnonymousEnumMembers) $(D })
@@ -249,7 +250,7 @@ enum
 }
 ---
 
-$(H2 $(LNAME2 manifest_constants, Manifest Constants))
+$(H3 $(LNAME2 single_member, Single Member Syntax))
 
         $(P If there is only one member of an anonymous enum, the `{ }` can
         be omitted. Gramatically speaking, this is an $(GLINK2 declaration, AutoDeclaration).
@@ -260,6 +261,10 @@ enum i = 4;      // i is 4 of type int
 enum long l = 3; // l is 3 of type long
 ---
 
+$(H2 $(LNAME2 manifest_constants, Manifest Constants))
+
+        $(P Enum members are manifest constants, which exist only at compile-time.)
+
         $(P Manifest constants are not lvalues, meaning their address
         cannot be taken.  They exist only in the memory of the compiler.)