refactor(core): polish failure messages for template-var-assignment schematic (#29708)

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

packages/core/schematics/migrations/template-var-assignment/README.md

@@ -0,0 +1,27 @@
+## Assignments to template variables
+
+With Ivy, assignments to template variables are no longer supported
+as template variables are effectively constants.
+
+This means that assignments to template variables will break your
+application once Ivy is enabled by default. For example:
+
+```html
+<button *ngFor="let option of options"
+       (click)="option = 'newButtonText'">
+  {{ option }}
+</button>
+```
+
+In the example from above, a value is assigned to the `option`
+template variable on `click`. This will ultimately break your
+application and therefore the logic needs to be adjusted to not
+update the `option` variable, but rather the given element in
+the `options` array:
+
+```html
+<button *ngFor="let option of options; let idx = index"
+       (click)="options[idx] = 'newButtonText'">
+  {{ option }}
+</button>
+```