docs: Add more documentation to OdbBackend

Added more documentation to clarify how to do error handling properly.

Clarified the default behaviour of `OdbBackend::exists`.

Author: tecc

src/odb_backend.rs

@@ -11,10 +11,18 @@ use std::{ptr, slice};
 
 /// A custom implementation of an [`Odb`] backend.
 ///
-/// # Errors
+/// Most of the default implementations of this trait's methods panic when called as they are
+/// intended to be overridden.
 ///
-/// If the backend does not have enough memory, the error code should be
-/// [`ErrorCode::GenericError`] and the class should be [`ErrorClass::NoMemory`].
+/// # Error recommendations
+///
+/// Errors are generally left at the implementation's discretion; some recommendations are
+/// made regarding error codes and classes to ease implementation and usage of custom backends.
+///
+/// Read the individual methods' documentation for more specific recommendations.
+///
+/// If the backend does not have enough memory, the error SHOULD be code
+/// [`ErrorCode::GenericError`] and the class SHOULD be [`ErrorClass::NoMemory`].
 #[allow(unused_variables)]
 pub trait OdbBackend {
     /// Returns the supported operations of this backend.
@@ -163,6 +171,8 @@ pub trait OdbBackend {
     /// Requires that [`SupportedOperations::EXISTS`] is present in the value returned from
     /// [`supported_operations`] to expose it to libgit2.
     ///
+    /// The default implementation of this method panics.
+    ///
     /// # Implementation notes
     ///
     /// An implementation SHOULD return `Ok(true)` if the object exists, and `Ok(false)` if the