Clarify that there is no RCS for rgba() or hsla()

[svgeesus]

@weinig noted on wpt:

Testing that hsla(from ...) and rgba(from ...) are not valid in the relative color syntax (but rgb(from ...) and hsl(from ...) are). Would like confirmation that this is correct

Confirmed, see the grammar at rgb(from and hsl(from. However, I will add examples with non-unity alpha just to make that clearer.

[triple-underscore]

The wording "rgba legacy color syntax" in the newly added example seems misleading, since the legacy color syntax can be used with both "non-legacy RGB syntactic form" rgb() and "legacy RGB syntactic form" rgba().

[svgeesus]

No, legacy color syntax covers three things, of which a separate function for alpha is only one.

[triple-underscore]

Ah, I reread the definition, and realized that I had misunderstood it, sorry.

[svgeesus]

No problem.