[css-font-loading] Add a clarifying note to the 'UAs can load a FontFace whenever they feel like' paragraph.

Author: tabatkins

css-font-loading/Overview.bs

@@ -336,6 +336,16 @@ The <code>load()</code> method</h3>
 	When this happens,
 	they must act as if they had called the corresponding {{FontFace}}’s {{FontFace/load()}} method described here.
 
+	Note: Some UAs utilize a "font cache"
+	which avoids having to download the same font multiple times
+	on a page or on multiple pages within the same origin.
+	Multiple {{FontFace}} objects can be mapped to the same entry in the font cache,
+	which means that a {{FontFace}} object might start loading unexpectedly,
+	even if it's not in a {{FontFaceSet}},
+	because some other {{FontFace}} object pointing to the same font data
+	(perhaps on a different page entirely!)
+	has been loaded.
+
 <h3 id='font-face-css-connection'>
 Interaction with CSS’s ''@font-face'' Rule</h3>