Make it more clear that form names need to be unique

[jakzal]

Currently, the docs say:

This new class contains all the directions needed to create the task form (note that the getName() method should return a unique identifier for this form "type").

I propose it's made more visible the name should be unique. It's no where verified, and might lead to unexpected problems (like symfony/symfony#13590).

[javiereguiluz]

Do you really think that this is a documentation problem? I think that obscure problems like this should be reported using an exception thrown by the Form component.

[jakzal]

I don't think it's trivial to solve in the code (ping @webmozart).

I agree it's not really user friendly at the moment. However, even if we solved it in the code, this is an important detail, which should be highlighted in the docs in my opinion.

[wouterj]

It's already highlighted: "should return a unique identifier"

I don't think we can solve it better. Instead, I agree with @javiereguiluz, the code should have some validation for it to also help people who don't read the docs.

[jakzal]

@wouterj it's not highlighted. It doesn't stand out visually ;) I agree we should do it in code too, but fixing docs is a quick win.

[Nek-]

👍 i notice beginners like make many types that have same names. More than that, they name them like symfony types.

[stof]

@wouterj allowing to overwrite a form type by registering a type again with this name is actually a feature.

[wouterj]

Closing as #5076 added a caution, which is the most clear way of warning the user in the docs.