docs(input[radio]): explain what happens with same name on multiple inputs

[gkalpak]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Docs update.

Does this PR introduce a breaking change?
No

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Fix/Feature: Docs have been added/updated
• Fix/Feature: Tests have been added; existing tests pass

Other information:
Closes #15009.

[gkalpak]

This is what it looks like:

[Narretz]

2 things we should note:

• Since AngularJS uses ngModel to set the radio state, and not the name, the radio state is not constrained to the parent form, but to the scope the ngModel is in.
• You can still give the radios in a group the same name, but if you need to access each ngModelController individually, you cannot do it

[gkalpak]

I do mention that the state of the radion button is determined based on the model data. I am afraid that saying something like "the radio state is not constrained to the parent form, but to the scope the ngModel is in" would sound unnecessarily complicated.

I will expand on what happens when using the same name for multiple radios.

[Narretz]

Ok. It's implicit that when the model changes the checked status, it's not restricted to the parent form.