Clarify #named-access-on-the-window-object

[delan]

This patch clarifies #named-access-on-the-window-object per #9068 by:

• adding a note that reads “Since the Window interface has the [Global] extended attribute, its named properties follow the rules for named properties objects rather than legacy platform objects.”
• in the IDL for #window, reordering the lines near the named getter to make the comment easier to notice
• elaborating on the note that reads “There will be at least one such object, by definition.” because it wasn’t clear how this was the case or which definition guaranteed that (at least without reading the source)
• elaborating on the similar note in #dom-document-nameditem for the same reason

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/dom.html ( diff )
/infrastructure.html ( diff )
/nav-history-apis.html ( diff )

[domenic]

Looks good with some typographical fixes