Document effect of type aliases on global level (#42666)

Adds the sentence @JeffBezanson proposed to the documentation and solves #30824.

doc/src/manual/performance-tips.md

@@ -14,8 +14,8 @@ The functions should take arguments, instead of operating directly on global var
 ## Avoid global variables
 
 A global variable might have its value, and therefore its type, change at any point. This makes
-it difficult for the compiler to optimize code using global variables. Variables should be local,
-or passed as arguments to functions, whenever possible.
+it difficult for the compiler to optimize code using global variables. This also applies to type-valued variables, i.e. type aliases on the global level.
+Variables should be local, or passed as arguments to functions, whenever possible.
 
 
 We find that global names are frequently constants, and declaring them as such greatly improves