[css-text-4] Clarify fonts without enough blank may not be trimmed (w3c#9225)

* text-spacing-trim-give-up

* frivoal review

* Fix typo.

Author: kojiishi

css-text-4/Overview.bs

@@ -10625,6 +10625,13 @@ Fullwidth Punctuation Collapsing</h4>
 	the font has proportional ideographs
 	and the fullwidth advance width cannot be reliably determined by measuring glyphs.
 
+	Some fonts have fullwidth punctuation characters
+	whose blank are too small to trim (kern.)
+	UA may choose not to trim (kern)
+	when the UA determined the trimming (kerning) may cause
+	glyph cut-offs, collisions, or excessive kerning
+	through glyph bounding box, glyph metrics, or font features.
+
 	If 'text-spacing-trim' is ''trim-all'',
 	the UA must collapse the space typically associated with such full width glyphs
 	regardless of the context in which they appear.