docs(doctests+coverage): add documentation and doctests for all modules of secret-memory except for alloc

secret-memory/src/file.rs

@@ -1,8 +1,46 @@
+//! Objects that implement this Trait provide a way to store their data in way that respects the
+//! confidentiality of its data. Specifically, an object implementing this Trait guarantees
+//! if its data with [store_secret](StoreSecret::store_secret) are saved in the file with visibility
+//! equivalent to [rosenpass_util::file::Visibility::Secret].
+
 use std::path::Path;
 
+/// Objects that implement this Trait provide a standard method to be stored securely. The trait can
+/// be implemented as follows for example:
+/// # Example
+/// ```rust
+/// use std::io::Write;
+/// use std::path::Path;
+/// use rosenpass_secret_memory::file::StoreSecret;
+///
+/// use rosenpass_util::file::{fopen_w, Visibility};
+///
+/// struct MyWeirdI32 {
+///     _priv_i32: [u8; 4],
+/// }
+///
+/// impl StoreSecret for MyWeirdI32 {
+///     type Error = std::io::Error;
+///
+///     fn store_secret<P: AsRef<Path>>(&self, path: P) -> Result<(), Self::Error> {
+///         fopen_w(path, Visibility::Secret)?.write_all(&self._priv_i32)?;
+///         Ok(())
+///     }
+///
+///     fn store<P: AsRef<Path>>(&self, path: P) -> Result<(), Self::Error> {
+///         fopen_w(path, Visibility::Public)?.write_all(&self._priv_i32)?;
+///         Ok(())
+///     }
+/// }
+/// ```
 pub trait StoreSecret {
     type Error;
 
+    /// Stores the object securely. In particular, it ensures that the visibility is equivalent to
+    /// [rosenpass_util::file::Visibility::Secret].
     fn store_secret<P: AsRef<Path>>(&self, path: P) -> Result<(), Self::Error>;
+
+    /// Stores the object. No requirement on the visibility is given, but it is common to store
+    /// the data with visibility equivalent to [rosenpass_util::file::Visibility::Public].
     fn store<P: AsRef<Path>>(&self, path: P) -> Result<(), Self::Error>;
 }