With a script. You seem to be trying to bind an actual tag value to deliver to a tag's description property. Bindings cannot do that. You should reconsider your design--Ignition expects the description property of a tag to be the source of truth, and bindings to retrieve that are supported.
At best, you can use a valueChange event script on the memory tag to write to the other tag's attribute. You cannot use a binding.
Is it even possible?
Yes.
system.tag.writeBlocking(["[default]Foo.Documentation"], ["my docs"])
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How do I do use that?
You should watch some IU videos to have some orientation in the software.
Kevin wrote a script, which you can call from dozens of different places in the software to do what you want. We think, because what you're asking for isn't all that clear. If you want to do it once - just do it once via the tag editor.
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I need to do it hundreds of times.
Before you go down this road, you should plan to not have this broken architecture when you are done.
In the short term, you should add a value change script to the UDT definition's documentation memory tag that writes to the scaled value's documentation configuration. Apply that, so the text will actually move. Then delete the documentation memory tag in that UDT definition.
The script will be similar to what Kevin posted, but will use actual jython variables instead of example text.
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All of those memory tag values should be inside of UDT parameters 
. Having them inside memory tags will just cause you pain. If you're using those me tags in bindings, then bind their values inside the UDT definition to the new UDT parameters.
Then you can also just bind your value pv tags description to the parameter value as well. It would solve all of your current issues and more.
You should also do the free inductive university training, as it seems as though you're missing some fundamental understanding