Minor documentation update for AuxStore and BlockImportParams.auxiliary (#10428)

substrate/client/api/src/backend.rs

@@ -259,6 +259,10 @@ pub trait Finalizer<Block: BlockT, B: Backend<Block>> {
 }
 
 /// Provides access to an auxiliary database.
+///
+/// This is a simple global database not aware of forks. Can be used for storing auxiliary
+/// information like total block weight/difficulty for fork resolution purposes as a common use
+/// case.
 pub trait AuxStore {
 	/// Insert auxiliary data into key-value store.
 	///

substrate/client/consensus/common/src/block_import.rs

@@ -190,8 +190,9 @@ pub struct BlockImportParams<Block: BlockT, Transaction> {
 	/// rejects block import if there are still intermediate values that remain unhandled.
 	pub intermediates: HashMap<Cow<'static, [u8]>, Box<dyn Any + Send>>,
 	/// Auxiliary consensus data produced by the block.
-	/// Contains a list of key-value pairs. If values are `None`, the keys
-	/// will be deleted.
+	/// Contains a list of key-value pairs. If values are `None`, the keys will be deleted. These
+	/// changes will be applied to `AuxStore` database all as one batch, which is more efficient
+	/// than updating `AuxStore` directly.
 	pub auxiliary: Vec<(Vec<u8>, Option<Vec<u8>>)>,
 	/// Fork choice strategy of this import. This should only be set by a
 	/// synchronous import, otherwise it may race against other imports.