[css-text] Add informal summary of text-spacing-trim

Part of #9511

css-text-4/Overview.bs

@@ -10570,6 +10570,47 @@ CJK Punctuation Spacing: the 'text-spacing-trim' property</h3>
 			ISSUE: Do we need ''text-spacing-trim/auto''? It would be weird for the author to choose platform-dependent behavior at the start of the first line, and it should otherwise use ''text-spacing-trim/trim-auto''.
 	</dl>
 
+	<div class=note>
+		Here is an informal summary of what the various values do:
+
+		<table class="complex data">
+		<thead>
+			<tr>
+				<th>Value
+				<th>Trim at line start
+				<th>Trim adjacent pairs
+				<th>Trim at line end
+				<th>Trim everywhere
+		<tbody>
+			<tr>
+				<th>''space-all''
+				<td colspan=4>no
+			<tr>
+				<th>''text-spacing-trim/normal''
+				<td>no
+				<td rowspan=4>yes
+				<td>only if would not fit
+				<td rowspan=4>no
+			<tr>
+				<th>''trim-auto''
+				<td rowspan=2>yes
+				<td>yes
+			<tr>
+				<th>''trim-start''
+				<td>only if would not fit
+			<tr>
+				<th>''space-first''
+				<td>yes except on the first line
+				<td>yes
+			<tr>
+				<th>''trim-all''
+				<td colspan=4>yes
+			<tr>
+				<th>''auto''
+				<td colspan=4>user-agent specific / platform dependent
+		</table>
+	</div>
+
 
 <h4 id="fullwidth-collapsing">
 Fullwidth Punctuation Collapsing</h4>