Add comment regarding block sizes.

src/uu/sum/BENCHMARKING.md

@@ -13,11 +13,11 @@ $ cargo build --release --package uu_sum
 and then you can time how it long it takes to checksum the file by running
 
 ```shell
-$ /usr/bin/time ./target/release/sum wikidatawiki-20211001-pages-logging.xml
+$ time ./target/release/sum wikidatawiki-20211001-pages-logging.xml
 ```
 
 For more systematic measurements that include warm-ups, repetitions and comparisons, [Hyperfine](https://github.com/sharkdp/hyperfine) can be helpful. For example, to compare this implementation to the one provided by your distribution run
 
 ```shell
-$ hyperfine "./target/release/sum wikidatawiki-20211001-pages-logging.xml" "/usr/bin/sum wikidatawiki-20211001-pages-logging.xml"
+$ hyperfine "./target/release/sum wikidatawiki-20211001-pages-logging.xml" "sum wikidatawiki-20211001-pages-logging.xml"
 ```

src/uu/sum/src/sum.rs

@@ -45,6 +45,7 @@ fn bsd_sum(mut reader: Box<dyn Read>) -> (usize, u16) {
         }
     }
 
+    // Report blocks read in terms of 1024-byte blocks.
     let blocks_read = div_ceil(bytes_read, 1024);
     (blocks_read, checksum)
 }
@@ -69,6 +70,7 @@ fn sysv_sum(mut reader: Box<dyn Read>) -> (usize, u16) {
     ret = (ret & 0xffff) + (ret >> 16);
     ret = (ret & 0xffff) + (ret >> 16);
 
+    // Report blocks read in terms of 512-byte blocks.
     let blocks_read = div_ceil(bytes_read, 512);
     (blocks_read, ret as u16)
 }