[css-grid-1] Clarify that fit-content() substitution rule only applies to its max track sizing function. (We are not treating fit-content()'s min track sizing function as max-content ever! Related to w3c#3621

Author: fantasai

css-grid-1/Overview.bs

@@ -4271,9 +4271,11 @@ Distributing Extra Space Across Spanned Tracks</h4>
 							all affected tracks.
 					</ul>
 
-					For this purpose, ''fit-content()'' tracks are treated as ''max-content''
-					until they reach the limit specified as the ''fit-content()'' argument,
-					after which they are treated as having a <a>fixed sizing function</a> of that argument.
+					For this purpose,
+					the [=max track sizing function=] of a ''fit-content()'' track
+					is treated as ''max-content''
+					until it reaches the limit specified as the ''fit-content()'' argument,
+					after which it is treated as having a <a>fixed sizing function</a> of that argument.
 
 				<li>
 					For each affected track,