zcash_client_backend: Add further doc comments and revert default fea…

…tures

zcash_client_backend/Cargo.toml

@@ -116,8 +116,6 @@ tokio = { version = "1.21.0", features = ["rt-multi-thread"] }
 time = ">=0.3.22, <0.3.24" # time 0.3.24 has MSRV 1.67
 
 [features]
-default = ["unstable", "orchard", "sync"]
-
 ## Enables the `tonic` gRPC client bindings for connecting to a `lightwalletd` server.
 lightwalletd-tonic = ["dep:tonic"]
 

zcash_client_backend/src/data_api/chain.rs

@@ -382,19 +382,26 @@ pub trait BlockCache: BlockSource + Send + Sync {
     /// indicating there are blocks missing from the cache.
     fn read(&self, range: &ScanRange) -> Result<Vec<CompactBlock>, Self::Error>;
 
-    /// Returns the height of highest block known to the block cache within a specified range.
+    /// Finds the height of the highest block known to the block cache within a specified range.
     /// If `range` is `None`, returns the tip of the entire cache.
-    /// If no blocks are found in the cache, returns `None`.
+    ///
+    /// Returns `Ok(Some(BlockHeight))` on success, otherwise returns an error.
+    /// If no blocks are found in the cache, returns Ok(`None`).
     fn get_tip_height(&self, range: Option<&ScanRange>)
         -> Result<Option<BlockHeight>, Self::Error>;
 
     /// Inserts a vec of compact blocks into the block cache.
+    ///
+    /// Returns `Ok(())` on success, otherwise returns an error.
     fn insert(&self, compact_blocks: Vec<CompactBlock>) -> Result<(), Self::Error>;
 
     /// Removes all cached blocks above a specified block height.
+    ///
+    /// Returns `Ok(())` on success, otherwise returns an error.
     fn truncate(&self, block_height: BlockHeight) -> Result<(), Self::Error>;
 
     /// Deletes a range of compact blocks from the block cache.
+    ///
     /// Returns a `JoinHandle` from a `tokio::spawn` task.
     fn delete(&self, range: &ScanRange) -> JoinHandle<Result<(), Self::Error>>;
 }