add note about references in Out (#49729)

Author: MasonProtter

stdlib/REPL/docs/src/index.md

@@ -646,6 +646,13 @@ Out[3]: Dict{Int64, Any} with 2 entries:
   1 => 8
 ```
 
+!!! note
+    Since all outputs from previous REPL evaluations are saved in the `Out` variable, one should be careful if they are returning many
+    large in-memory objects like arrays, since they will be protected from garbage collection so long as a reference to them remains in
+    `Out`. If you need to remove references to objects in `Out`, you can clear the entire history it stores with `empty!(Out)`, or clear
+    an individual entry with `Out[n] = nothing`.
+
+
 ## TerminalMenus
 
 TerminalMenus is a submodule of the Julia REPL and enables small, low-profile interactive menus in the terminal.