Expand and clarify the __mode metaproperty

content/en-us/luau/metatables.md

@@ -111,7 +111,7 @@ Here's the list of available metamethods:
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 		  <td>`__mode`</td>
-		  <td>Used in weak tables, declaring whether the keys and/or values of a table are weak. Note that references to Roblox instances are never weak. Tables that hold such references will never be garbage collected.</td>
+		  <td>A string making either all of the table's keys (`"k"`), values (`"v"`), or both (`"kv"`) weak. This means that the garbage collector will not count them as references, allowing them to be freed from memory when they're no longer used anywhere else. When a table entry's key or value (as described by the string) is garbage collected, it will be removed since one them is `nil` (or possibly both if using `"kv"`) and the entry will no longer exist. Only tables, functions, buffers (`Library.buffer`), and some [userdata](./userdata.md) are garbage collectable; other [types](./index.md#types) are not and won't be affected. Note that even if you don't explicitly hold a reference to an object in your code, the engine might still be keeping one, for example if a Roblox `Class.Instance` is a descendant of the `Class.DataModel`.</td>
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 		  <td>`__len(table)`</td>