Editorial: consolidate dictionary default values paragraphs

This makes it easier to understand what values are actually allowed as default values.

Author: jyasskin

index.bs

@@ -4752,11 +4752,6 @@ supplied as an argument to an [=operation=]) to an IDL dictionary. Specification
 leave the members [=dictionary member/optional=] in all other cases, including when a dictionary
 type is used solely as the [=return type=] of [=operations=].
 
-[=dictionary member/Optional=] [=dictionary members=] can also be specified as having a
-<dfn id="dfn-dictionary-member-default-value" for="dictionary member" export>default value</dfn>,
-which is the value used by default when author code or specification text does not provide a value
-for that member.
-
 A given dictionary value of type |D| can have [=map/entries=] for each of the dictionary members
 defined on |D| and on any of |D|'s [=inherited dictionaries=]. Dictionary members that are specified
 as [=dictionary member/required=], or that are specified as having a
@@ -4824,9 +4819,12 @@ its [=nullable types/inner type=] must not be a [=dictionary type=].
     };
 </pre>
 
-If the identifier is followed by a <span class="char">U+003D EQUALS SIGN ("=")</span>
-and a value (matching <emu-nt><a href="#prod-DefaultValue">DefaultValue</a></emu-nt>),
-then that gives the dictionary member its [=dictionary member/default value=].
+If the identifier for an [=dictionary member/optional=] [=dictionary member=] is followed by a <span
+class="char">U+003D EQUALS SIGN ("=")</span> and a value (matching <emu-nt><a
+href="#prod-DefaultValue">DefaultValue</a></emu-nt>), then that gives the dictionary member its <dfn
+id="dfn-dictionary-member-default-value" for="dictionary member" export>default value</dfn>, which
+is the value used by default when author code or specification text does not provide a value for
+that member.
 
 <pre highlight="webidl" class="syntax">
     dictionary identifier {