Adds info that MC_HOST envvar uses s3v4 api (#1393)

Closes #1379

Not staged.

Author: djwfyi

source/reference/minio-mc/minio-client-settings.rst

@@ -25,6 +25,8 @@ Host Credentials
 Use this setting to add a temporary alias to use for `mc` commands.
 For example, for use with scripting.
 
+The temporary alias uses the :aws-docs:`AWS s3v4 API <AmazonS3/latest/API/sig-v4-authenticating-requests.html>`.
+
 .. tab-set::
 
    .. tab-item:: Environment Variable