Port global var description to let (#30388)

* Port global var description to let

* Update files/en-us/web/javascript/reference/statements/let/index.md

Co-authored-by: Joshua Chen <[email protected]>

---------

Co-authored-by: Hamish Willee <[email protected]>

Author: Josh-Cena

files/en-us/web/javascript/reference/statements/let/index.md

@@ -39,7 +39,10 @@ The scope of a variable declared with `let` is one of the following curly-brace-
 - Function body
 - [Static initialization block](/en-US/docs/Web/JavaScript/Reference/Classes/Static_initialization_blocks)
 
-Or the current module or script, if it's contained in none of these.
+Or if none of the above applies:
+
+- The current [module](/en-US/docs/Web/JavaScript/Guide/Modules), for code running in module mode
+- The global scope, for code running in script mode.
 
 Compared with {{jsxref("Statements/var", "var")}}, `let` declarations have the following differences:
 
@@ -104,6 +107,8 @@ This differs from using `typeof` for undeclared variables, and variables that ho
 console.log(typeof undeclaredVariable); // "undefined"
 ```
 
+> **Note:** `let` and `const` declarations are only processed when the current script gets processed. If you have two `<script>` elements running in script mode within one HTML, the first script is not subject to the TDZ restrictions for top-level `let` or `const` variables declared in the second script, although if you declare a `let` or `const` variable in the first script, declaring it again in the second script will cause a [redeclaration error](#redeclarations).
+
 ### Redeclarations
 
 `let` declarations cannot be in the same scope as any other declaration, including `let`, {{jsxref("Statements/const", "const")}}, {{jsxref("Statements/class", "class")}}, {{jsxref("Statements/function", "function")}}, {{jsxref("Statements/var", "var")}}, and {{jsxref("Statements/import", "import")}} declaration.