scylla/Cargo.toml: enable missing-docs lint

This commit enables the `missing-docs` lint in the `scylla` crate.
Since now, the `scylla` crate will emit warnings for any public items
that do not have documentation comments.
This will hopefully help us to improve the documentation coverage
in the project.

Author: wprzytula

scylla/Cargo.toml

@@ -140,6 +140,7 @@ required-features = ["unstable-testing"]
 [lints.rust]
 unnameable_types = "warn"
 unreachable_pub = "warn"
+missing-docs = "warn"
 unexpected_cfgs = { level = "warn", check-cfg = [
     'cfg(scylla_cloud_tests)',
     'cfg(cassandra_tests)',