[TwigComponent] explain that default values are mandatory for props

[alexislefebvre]

Q	A
Bug fix?	no
New feature?	no
Deprecations?	no
Documentation?	yes
Issues	-
License	MIT

I wanted to write {% props icon, type %} but it didn't work, because default values are mandatory for props.

So this PR make it explicit.

[Kocal]

This is not true, properties do not requires default values.
If you define a property without a default value and that you forgot to use it, the exception <prop> should be defined for component components/<component>.html.twig is raised.

[alexislefebvre]

This is not true, properties do not requires default values. If you define a property without a default value and that you forgot to use it, the exception <prop> should be defined for component components/<component>.html.twig is raised.

Thanks, this is a better way to say it. If I update this PR with this content, would you consider reopening and merging it?

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Update: I’ll try to take this into account in #3132