ID-56: amend documentation

Author: mlookaxw

doc/manual/_config-tool.adoc

@@ -59,7 +59,7 @@ Options:
   -b DIRECTORY, --base-dir=DIRECTORY
                         Base directory for certificate files [multiple]
   --secrets-file=FILEPATH
-                        Path of JSON file containing confidential propertiers
+                        Path of JSON file containing confidential properties
                         [optional]
   --secrets-key=FILEPATH
                         Path to key file to decrypt confidential properties [optional]
@@ -91,6 +91,9 @@ Multiple files are supported by specifying the parameter multiple times.
 
 If a property occurs in multiple files the value from the last file is used.
 
+Specify a single dash '-' as filename to read properties from `stdin`.
+The input must be a flat key/value JSON document (e.g., `{"p1": "v1", "p2": "v2"}`).
+
 |--cert
 |An optional JSON file containing the certificate configuration.