base64: clarify example descriptions that do not edit files inplace (#5656)

pages/common/base64.md

@@ -3,11 +3,11 @@
 > Encode or decode file or standard input to/from Base64, to standard output.
 > More information: <https://www.gnu.org/software/coreutils/base64>.
 
-- Encode a file:
+- Encode the contents of a file as base64 and write the result to stdout:
 
 `base64 {{filename}}`
 
-- Decode a file:
+- Decode the base64 contents of a file and write the result to stdout:
 
 `base64 --decode {{filename}}`