WIP: Clarify performance penalty of global (JuliaLang#42452)

* Clarify performance penalty of global

I've slightly changed the wording for why a global creates a penalty in performance, as it raised confusion [such as here](https://discourse.julialang.org/t/global-variables-const-by-default/33138/3?) and I think the current wording was wrong (changing the value does not need to change the type, correct?)

However, I am still not quite convinced as a variable *doesn't* has a type and therefore "it's type changes" seems a bit weird, doesn't it?

* Update performance-tips.md

* Update performance-tips.md

* Update performance-tips.md

* Update performance-tips.md

* Update performance-tips.md

* Update doc/src/manual/performance-tips.md

Co-authored-by: Viral B. Shah <[email protected]>
Co-authored-by: Fredrik Ekre <[email protected]>

Author: 3 people

doc/src/manual/performance-tips.md

@@ -13,10 +13,9 @@ 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. 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.
-
+A global variable might have its value, and therefore possibly its type, changed at any point. This makes
+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
 performance: