Add documentation about the semantics of `docker login`

Fixes #10550

Signed-off-by: Jesse Dearing <jesse.dearing@gmail.com>

docs/man/docker-login.1.md

@@ -17,6 +17,9 @@ Register or Login to a docker registry server, if no server is
 specified "https://index.docker.io/v1/" is the default. If you want to
 login to a private registry you can specify this by adding the server name.
 
+This stores encoded credentials in `$HOME/.dockercfg` on Linux or `%USERPROFILE%/.dockercfg`
+on Windows.
+
 # OPTIONS
 **-e**, **--email**=""
    Email