? kleene macro operator is unstable

[ehuss]

As noted in PR #424, the ? operator is still unstable. I'm wondering if it should stay (since unstable things usually aren't document). It looks like it may get stabilized soonish, so maybe it doesn't matter? (tracking issue: rust-lang/rust#48075)

I've written the grammar for macros which includes this, but I'm unsure if I should continue the trend of keeping this unstable syntax.

[ehuss]

This is now stabilized in rust-lang/rust#56245.