Improves the failure messages for the `template-var-assignment` schematic. After manual
testing of the schematic it's not quite clear for developers what the failure message means
without any context. The schematic now also references a short markdown file mentioning
what needs to be changed, but eventually this document needs to be expanded with more
information and context of the reasoning behind this change within Ivy.
PR Close#29708
Paul Gschwendtner