Reinstate legacy encoding information (#1504)

Fixes #1039

sections/iana.include

@@ -37,8 +37,11 @@
       :: The <code>charset</code> parameter may be provided to specify the
           <a>document's character encoding</a>, overriding any
           [=character encoding declarations=] in the document other than a Byte Order Mark (BOM).
-          The parameter's value must be an <a>ASCII case-insensitive</a> match for the string
+          For newly created documents, the parameter's value must be an <a>ASCII case-insensitive</a> match for the string
           "<code>utf-8</code>".
+          For legacy documents, the character encoding name given must be an 
+          <a>ASCII case-insensitive</a> match for one of the <a lt="character encoding">labels</a> 
+          of the <a>character encoding</a> used to serialize the file. [[!ENCODING]]
   : Encoding considerations:
   :: 8bit (see the section on [=character encoding declarations=])
   : Security considerations:

sections/semantics-document-metadata.include

@@ -1417,7 +1417,7 @@
   Regardless of whether a character encoding declaration is present or not, the actual character
   encoding used to encode the document must be <a>UTF-8</a>. [[!ENCODING]]
 
-  The following restrictions apply to [=character encoding declarations=]:
+  The following restrictions apply to all [=character encoding declarations=]:
 
   * The character encoding declaration must be serialized without the use of
       <a>character references</a> or character escapes of any kind.
@@ -1426,6 +1426,10 @@
   * Due to a number of restrictions on <{meta}> elements, there can only be one
       <code>meta</code>-based character encoding declaration per document.
 
+  For legacy documents, the character encoding name given must be an <a>ASCII case-insensitive</a>
+  match for one of the <a lt="character encoding">labels</a> of the <a>character encoding</a> used 
+  to serialize the file. [[!ENCODING]]
+
   Authoring tools must default to using <a>UTF-8</a> for newly-created documents. [[!ENCODING]]
 
   If an <a>HTML document</a> does not start with a BOM, and its <a>encoding</a> is not explicitly