[css-anchor-position] Document why the allowed properties were chosen for that list. Remove corresponding issue.

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1448,13 +1448,16 @@ use of the ''revert'' or ''revert-layer'' values
 acts as if the property was part of the [=Author Origin=].
 (Thus, it instead reverts back to the [=User Origin=].)
 
-
-Issue(#9195): What exactly are the constraints that determine what's allowed here?
-Current list is based off of what's reasonable
-from Chrome's experimental impl.
-We can make a CQ that keys off of which fallback was used
-to allow more general styling,
-at least for descendants.
+Note: The [=accepted @position-try properties=] are the smallest group of properties
+that affect just the size and position of the box itself,
+without changing its contents area.
+This significantly simplifies the implementation of position fallback,
+without reducing the possible behaviors overly much.
+It is expected that a future extension to [=container queries=]
+will allow querying an element
+based on the position fallback it's using,
+allowing many cases not allowed by this restricted list
+to be handled.
 
 Issue(#7758): Would be useful to be able to detect
 when your anchor(s) are fully off-screen