zcash_client_backend: Fix `scan_cached_blocks` example doc compilation errors.

zcash_client_backend/src/data_api/chain.rs

@@ -58,9 +58,12 @@
 //!     // the first element of the vector of suggested ranges.
 //!     match scan_ranges.first() {
 //!         Some(scan_range) if scan_range.priority() == ScanPriority::Verify => {
+//!             // Download the chain state for the block prior to the start of the range you want
+//!             // to scan.
+//!             let chain_state = unimplemented!("get_chain_state(scan_range.block_range().start - 1)?;");
 //!             // Download the blocks in `scan_range` into the block source, overwriting any
 //!             // existing blocks in this range.
-//!             unimplemented!();
+//!             unimplemented!("cache_blocks(scan_range)?;");
 //!
 //!             // Scan the downloaded blocks
 //!             let scan_result = scan_cached_blocks(
@@ -68,6 +71,7 @@
 //!                 &block_source,
 //!                 &mut wallet_db,
 //!                 scan_range.block_range().start,
+//!                 chain_state,
 //!                 scan_range.len()
 //!             );
 //!
@@ -118,6 +122,9 @@
 //! //    encountered, this process should be repeated starting at step (3).
 //! let scan_ranges = wallet_db.suggest_scan_ranges().map_err(Error::Wallet)?;
 //! for scan_range in scan_ranges {
+//!     // Download the chain state for the block prior to the start of the range you want
+//!     // to scan.
+//!     let chain_state = unimplemented!("get_chain_state(scan_range.block_range().start - 1)?;");
 //!     // Download the blocks in `scan_range` into the block source. While in this example this
 //!     // step is performed in-line, it's fine for the download of scan ranges to be asynchronous
 //!     // and for the scanner to process the downloaded ranges as they become available in a
@@ -125,7 +132,7 @@
 //!     // appropriate, and for ranges with priority `Historic` it can be useful to download and
 //!     // scan the range in reverse order (to discover more recent unspent notes sooner), or from
 //!     // the start and end of the range inwards.
-//!     unimplemented!();
+//!     unimplemented!("cache_blocks(scan_range)?;");
 //!
 //!     // Scan the downloaded blocks.
 //!     let scan_result = scan_cached_blocks(
@@ -133,6 +140,7 @@
 //!         &block_source,
 //!         &mut wallet_db,
 //!         scan_range.block_range().start,
+//!         chain_state,
 //!         scan_range.len()
 //!     )?;
 //!