Add note that xpcall's error handler preserves the trace (#944)

## Changes

Adds a note that xpcall's error handler preserves the stack of the
erroring function.

This is useful to know if people are building error handlers and need to
know the traceback.

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---------

Author: metatablecat

content/en-us/reference/engine/globals/LuaGlobals.yaml

@@ -756,6 +756,10 @@ functions:
       succeeds without errors. In this case, `xpcall()` also returns all results
       from the call, after this first result. In case of any error, `xpcall()`
       returns false plus the result from `err`.
+
+      Unlike `Global.LuaGlobals.pcall()`, the `err` function preserves the stack
+      trace of function `f`, which can be inspected using `Library.debug.info()`
+      or `Library.debug.traceback()`.
     parameters:
       - name: f
         type: function