Apply suggested changes from review

Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: dlibby-

css-env-1/Overview.bs

@@ -52,8 +52,16 @@ These "global" variables have both benefits and downsides versus cascading varia
 	can be retrieved via ''env()'',
 	whereas the element-specific nature of ''var()'' was not an appropriate place to pipe that information in.
 
-[=Environment variables=] can be either be scalar values or array-like values which require the
-author to provide 0-based indices into the available dimensions to refer to a specific value.
+Most [=environment variables=] will have a single value at a time.
+Some, however, are "indexed", representing multiple values at once,
+such as the sizes and positions of several distinct panes of content 
+in the ''viewport-segment-*'' variables.
+To refer to these indexed variables, one or more integers must be provided
+alongside the variable name,
+like ''viewport-segment-width 1 0'',
+to select a single value from the list or grid of possibilities,
+similar to selecting one element from a list in a traditional programming language
+with a syntax like <code>values[0]</code>.
 
 Environment Variables {#environment}
 ====================================