[css-grid] Make it clearer that fr units resolved against an indefini…

…te grid container count as 'intrinsically-sized', and so trigger baseline-alignment fallback. Fixes #3042.

css-grid-1/Overview.bs

@@ -1788,13 +1788,13 @@ Flexible Lengths: the ''fr'' unit</h4>
 	so they cannot be represented in or combined with other unit types in ''calc()'' expressions.
 
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 <h4 id='computed-track-list'>
@@ -3413,6 +3413,8 @@ Inline-axis Alignment: the 'justify-self' and 'justify-items' properties</h3>
 	that item does not participate in baseline alignment,
 	and instead uses its <a>fallback alignment</a>
 	as if that were originally specified.
+	For this purpose, <<flex>> track sizes count as “intrinsically-sized”
+	when the [=grid container=] has an [=indefinite=] size in the relevant axis.
 
 	Note: Whether the fallback alignment is used or not
 	does not change over the course of layout:
@@ -3434,6 +3436,8 @@ Block-axis Alignment: the 'align-self' and 'align-items' properties</h3>
 	that item does not participate in baseline alignment,
 	and instead uses its <a>fallback alignment</a>
 	as if that were originally specified.
+	For this purpose, <<flex>> track sizes count as “intrinsically-sized”
+	when the [=grid container=] has an [=indefinite=] size in the relevant axis.
 
 <h3 id='grid-align'>
 Aligning the Grid: the 'justify-content' and 'align-content' properties</h3>