[css-fonts-4] Clarify the behavior of the 'very small' block period, to

resolve #4108.

css-fonts-4/Overview.bs

@@ -2668,8 +2668,7 @@ for example, forcing all fonts to have a ''0s'' <a>block period</a>.
 
 	<dt><dfn>fallback</dfn>
 	<dd>
-		Gives the font face an extremely small <a>block period</a>
-		(''100ms'' or less is recommended in most cases)
+		Gives the font face a [=no-network=] <a>block period</a>
 		and a short <a>swap period</a>
 		(''3s'' is recommended in most cases).
 
@@ -2696,19 +2695,32 @@ for example, forcing all fonts to have a ''0s'' <a>block period</a>.
 
 	<dt><dfn>optional</dfn>
 	<dd>
-		Gives the font face an extremely small <a>block period</a>
-		(''100ms'' or less is recommended in most cases)
-		and a ''0s'' <a>swap period</a>.
-
-		If the font is not retrieved before the two durations expire,
+		Gives the font a [=no-network=] [=block period=] and ''0s'' [=swap period=],
+		plus some additional caveats.
+
+		A <dfn export for="block period">no-network</dfn> [=block period=]
+		indicates that the user agent <em>should</em> block long enough
+		to determine if the font is locally installed
+		or cached from a previous network request
+		before falling back,
+		effectively giving the font a very small [=block period=],
+		but should not wait for a font to actually come from a "real" network request.
+		(If the user agent knows it's on a device
+		where waiting for an uncached local font
+		would likely cause a significant delay,
+		it can fall back immediately instead.) 
+
+		Additionally, if the font is not retrieved before the two durations expire,
 		the user agent may choose to abort the font download,
 		or download it with a very low priority.
 		If the user agent believes it would be useful for the user,
 		it may avoid even starting the font download,
 		and proceed immediately to using a fallback font.
 
-		Note: In other words, the font is used if it's already downloaded and available,
-		but otherwise a fallback is used for the rest of the page's lifetime instead.
+		Note: In other words, 'optional' is meant to allow usage of local or already-downloaded-and-cached fonts
+		without noticeably delaying rendering for the user,
+		but otherwise fall back immediately;
+		in either case, the text will render quickly and never re-render in a different font as a result of fonts loading.
 		The font might download in the background and be available to future page loads,
 		but if the user-agent detects that the user has very limited bandwidth,
 		it might choose to simply never download and use the font.
@@ -2726,7 +2738,8 @@ for example, forcing all fonts to have a ''0s'' <a>block period</a>.
 			than they do about the finer points of its display,
 			and ''optional'' provides a good behavior for them.
 			If they return later,
-			the desired font faces might have finished downloading,
+			and the page has correctly used caching semantics for its font downloads,
+			the desired font faces might be available in the user agent's cache,
 			giving them the "intended" experience without slowing down
 			either their first or subsequent visits.