Fixes#31186. This commit adds more context about the behavior
of template reference variables in nested templates and moves
doc into concepts section.
PR Close#31195
This commit updates the docs examples to be compatible with the
`variable-name` tslint rule without requiring the
`allow-leading-underscore` and `allow-trailing-underscore` options.
This is in preparation of updating the docs examples `tslint.json` to
match the one generated for new Angular CLI apps in a future commit.
PR Close#38143
