docs: Clarify use of the base64 utility

The base64 utility has different options on both Darwin and Linux.
Adding clarification as to how the use of these utilities differ
on platforms.

docs/user-guide/secrets/index.md

@@ -124,9 +124,11 @@ $ kubectl create -f ./secret.yaml
 secret "mysecret" created
 ```
 
-**Encoding Note:** The serialized JSON and YAML values of secret data are encoded as
-base64 strings.  Newlines are not valid within these strings and must be
-omitted (i.e. do not use `-b` option of `base64` which breaks long lines.)
+**Encoding Note:** The serialized JSON and YAML values of secret data are
+encoded as base64 strings.  Newlines are not valid within these strings and must
+be omitted.  When using the `base64` utility on Darwin/OS X users should avoid
+using the `-b` option to split long lines.  Conversely Linux users *should* add
+the option `-w 0` to `base64` commands.
 
 #### Decoding a Secret