[css-align] Clarify that left/right are self-position values, they're just excluded from <self-position> for reasons. Fixes #1980.

Author: tabatkins

css-align-3/Overview.bs

@@ -338,7 +338,7 @@ Positional Alignment: the ''center'', ''start'', ''end'', ''self-start'', ''self
 			this value behaves as ''start''.
 	</dl>
 
-	Two grammar terms are used to denote this collection of values:
+	Two grammar terms are used to denote certain subsets of these values:
 
 	<dl>
 		<dt><<self-position>>
@@ -361,6 +361,11 @@ Positional Alignment: the ''center'', ''start'', ''end'', ''self-start'', ''self
 			</pre>
 	</dl>
 
+	Note: ''left'' and ''right'' are excluded from <<self-position>>,
+	despite being valid [=positional alignment=] values for 'justify-self',
+	because they are not allowed in 'align-self'.
+	They are instead explicitly included in the 'justify-self' grammar.
+
 	<aside class=example>
 		The ''<self-position>/start'' and ''<self-position>/end'' keywords
 		are <a>flow-relative</a>:
@@ -1110,7 +1115,7 @@ Baseline Content-Alignment</h3>
 	</dl>
 
 	Additionally, if <a>self-alignment</a> applies to the box in an axis,
-	in order to participate in <a>baseline content-alignment</a> in that axis 
+	in order to participate in <a>baseline content-alignment</a> in that axis
 	it must also have a <dfn>coordinated self-alignment preference</dfn>;
 	that is: