zeroize: gate custom derive rustdoc on the `derive` feature
Closes
Adds a `#[cfg(feature = "derive")]` to the `zeroize` doctests.
This allows the doctests to pass when the feature is disabled.
//! Example which derives `Drop`:
//!
//! ```
+//! # #[cfg(feature = "derive")]
+//! # {
//! use zeroize::Zeroize;
//!
//! // This struct will be zeroized on drop
//! #[derive(Zeroize)]
//! #[zeroize(drop)]
//! struct MyStruct([u8; 32]);
+//! # }
//! ```
//!
//! Example which does not derive `Drop` (useful for e.g. `Copy` types)
//!
//! ```
+//! #[cfg(feature = "derive")]
+//! # {
//! use zeroize::Zeroize;
//!
//! // This struct will *NOT* be zeroized on drop
//! #[derive(Copy, Clone, Zeroize)]
//! struct MyStruct([u8; 32]);
+//! # }
//! ```
//!
//! ## `Zeroizing<Z>`: wrapper for zeroizing arbitrary values on drop
