Fix #8015: update docs

[liufengyun]

Fix #8015: update docs

[unkarjedy]

@liufengyun
from docs

In Scala 3.0, both _ and ? are legal names for wildcards.
In Scala 3.1, _ is deprecated in favor of ? as a name for a wildcard. A -rewrite option is available to rewrite one to the other.
In Scala 3.2, the meaning of _ changes from wildcard to placeholder for type parameter.

Scala 3.1 and 3.2 will come not very soon, to put it mildly.
Maybe _ should be left in the grammar until then?

[liufengyun]

@unkarjedy there are other deprecated but supported syntaxes that are not reflected in the doc. Adding them to the doc will be confusing.

[nicolasstucki]

Though we should be clear on what syntax we support for migration modes