*sum: update pages to make them consistent (#14923)

* *sum: update pages to make them consistent

* shasum: remove redundant option

* shasum: update page

pages/common/b2sum.md

@@ -15,7 +15,7 @@
 
 `{{command}} | b2sum`
 
-- Read a file of BLAKE2 sums and filenames and verify all files have matching checksums:
+- Read a file of BLAKE2 checksums and filenames and verify all files have matching checksums:
 
 `b2sum --check {{path/to/file.b2}}`
 
@@ -26,3 +26,7 @@
 - Only show a message when verification fails, ignoring missing files:
 
 `b2sum --ignore-missing --check --quiet {{path/to/file.b2}}`
+
+- Check a known BLAKE2 checksum of a file:
+
+`echo {{known_blake2_checksum_of_the_file}} {{path/to/file}} | b2sum --check`