Clarification about for loop index variables

Describe the bug
Arising from support tickets (private):
https://support.microbit.org/helpdesk/tickets/88776
https://support.microbit.org/helpdesk/tickets/88841

Changing modes sometimes results in extra numbered variables being created, particularly with for loops.

For example, when there are two for loops, a round trip to JavaScript with a minor edit, changes the second index to index2, though it doesn't really need to be.
https://makecode.microbit.org/_3ALKHjemTc9L

Another case is when the same name appears both inside and outside a for loop.

• An existing variable can be selected as the index of a for loop https://makecode.microbit.org/_HAP1Ufe50feR
• Conversely, when a for loop is created, the index variable's name appears in toolbox/Variables, and can be dragged outside the loop. https://makecode.microbit.org/_5k5aAxPRPAdM

The behaviour is not obvious - it's as if there are two variables with the same name. It's clear in JavaScript mode that the loop index is a local that shadows the global inside the loop. It's less obvious in Python.

There is a subtle difference in appearance between variable blocks originating from the index variable and those created from toolbox/Variables. The loop index form disappears after a round trip with a minor edit in JavaScript mode. See both forms inside and outside the loop in this example.
https://makecode.microbit.org/_HAP1Ufe50feR

A round trip to Python can result in a for being replaced with a while.

Could the shadowed global be avoided - "that name already exists"?
Should the numbering happen sooner?
Should it be explained in documentation? https://makecode.microbit.org/blocks/loops/for

Reproduce the linked examples in Blocks, JavaScript, and Python modes, focusing on for-loop index variables, shadowed globals, and round trips. Compare the behavior with the linked for-loop documentation; completion depends on an agreed resolution for naming, scope, conversion, and documentation.