Rollup merge of #49985 - zackmdavis:0, r=estebank

Rollup merge of #49985 - zackmdavis:0, r=estebank

don't see issue #0

The unstable-feature attribute requires an issue (neglecting it is
E0547), which gets used in the error messages. Unfortunately, there are
some cases where "0" is apparently used a placeholder where no issue
exists, directing the user to see the (nonexistent) issue #0. (It would
have been better to either let `issue` be optional—compare to how issue
is an `Option<u32>` in the feature-gate declarations in
libsyntax/feature-gate.rs—or actually require that an issue be created.)
Rather than endeavoring to change how `#[unstable]` works at this time
(given competing contributor and reviewer priorities), this simple patch
proposes the less-ambitious solution of just not adding the "(see
issue)" note when the number is zero.

Resolves #49983.