[mediaqueries] Fill in the TODO describing boolean algebra.

Author: tabatkins

mediaqueries/Overview.bs

@@ -656,7 +656,47 @@ Combining Media Features {#media-conditions}
 	using full boolean algebra
 	(not, and, or).
 
-	Issue: TODO: Fill this in.
+	* Any media feature can be negated by placing ''not'' before it.
+		For example, ''not (color)'' inverts the meaning of ''(color)''--
+		since ''(color)'' matches a device with any kind of color display,
+		''not (color)'' matches a device <em>without</em> any kind of color display.
+
+	* Two or more media features can be chained together,
+		such that the query is only true if <em>all</em> of the media features are true,
+		by placing ''and'' between them.
+		For example, ''(width < 600px) and (height < 600px)''
+		only matches devices whose screens are smaller than ''600px'' wide in both dimensions.
+
+
+	* Alternately, two or more media features can be chained together,
+		such that the query is true if <em>any</em> of the media features are true,
+		by plaing ''or'' between them.
+		For example, ''(update: slow) or (hover: none)''
+		matches if the device is slow to update the screen (such as an e-reader)
+		<em>or</em> the device has no hover capability,
+		perhaps indicating that one should use a layout that displays more information
+		rather than compactly hiding it until the user hovers.
+
+	* <a>Media conditions</a> can be grouped by wrapping them in parentheses ''()''
+		which can then be nested within a condition the same as a single media query.
+		For example, ''not (color) or (hover)''
+		is true on devices that are monochrome
+		and/or that have hover capabilities,
+		because the ''not'' only applies to the immediately following ''(color)'' feature.
+		If one instead wanted to query for a device that was monochrome and <em>didn't</em> have hover capabilities,
+		it must instead be written as ''not ((color) or (hover))''
+		(or, equivalently, as ''not (color) and not (hover)'').
+
+	It is <em>invalid</em> to mix ''and'' and ''or'' at the same “level” of a media query.
+	For example, ''(color) and (pointer) or (hover)'' is illegal,
+	as it's unclear what was meant.
+	Instead, parentheses can be used to group things using a particular joining keyword,
+	yielding either ''(color) and ((pointer) or (hover))''
+	or ''((color) and (pointer)) or (hover)''.
+	These two have very different meanings:
+	if only ''(hover)'' is true,
+	the first one evaluates to false
+	but the second evaluates to true.
 
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