Clarify keystore add-file command behavior (#97474)

This commit adds a little more explanation to what add-file does on the
keystore.

relates #97459

docs/reference/commands/keystore.asciidoc

@@ -195,7 +195,8 @@ Values for multiple settings must be separated by carriage returns or newlines.
 
 You can add sensitive files, like authentication key files for Cloud plugins,
 using the `add-file` command. Settings and file paths are specified in pairs
-consisting of `setting path`.
+consisting of `setting path`. The value of the setting will be the binary contents
+of the file path at the time the file is added to the keystore.
 
 [source,sh]
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