Hide the Error type from the Documentation

In order to not commit to having a publicly visible copy of the Error
trait in snafu, hiding it from the documentation allows for snafu to
still support all of its features, but the user never gets to use the
trait. This way snafu can switch out the trait at any point with
`core::error::Error` if that becomes a thing. Alternatively the trait
here can also be made visible at some point. Both of these possibilities
are not breaking, so the conservative approach with doc(hidden) allows
for the smoothest experience going forward.

Cargo.toml

@@ -26,7 +26,7 @@ all-features = true
 [features]
 default = ["backtraces", "std"]
 
-# Uses the `std::error::Error` trait instead of its own
+# Implement the `std::error::Error` trait.
 std = []
 
 # Adds the backtrace type

src/lib.rs

@@ -132,11 +132,13 @@ generate_guide! {
 doc_comment::doctest!("../README.md", readme_tests);
 
 #[cfg(any(feature = "std", test))]
+#[doc(hidden)]
 pub use std::error::Error;
 
 #[cfg(not(any(feature = "std", test)))]
 mod no_std_error;
 #[cfg(not(any(feature = "std", test)))]
+#[doc(hidden)]
 pub use no_std_error::Error;
 
 /// Ensure a condition is true. If it is not, return from the function