clarified user binding (hopefully) (#48)

* clarified user binding

* realized that some of my clarification was obfuscation and tried again

* modified the example for successful binding

Author: GodloveD

pages/docs/admin-docs/advanced-config.md

@@ -46,7 +46,7 @@ The syntax for this consists of a bind path source and an optional bind path des
 ### USER BIND CONTROL (boolean, default='yes')
 In addition to the system bind points as specified within this configuration file, you may also allow users to define their own bind points inside the container. This feature is used via multiple command line arguments (e.g. `--bind`, `--scratch`, and `--home`) so disabling user bind control will also disable those command line options.
 
-Singularity will automatically disable this feature if the host does not support the prctl option `PR_SET_NO_NEW_PRIVS`.
+Singularity will automatically disable this feature if the host does not support the prctl option `PR_SET_NO_NEW_PRIVS`. In addition, `enable overlay` must be set to `yes` and the host system must support overlayFS (generally kernel versions 3.18 and later) for users to bind host directories to bind points that do not already exist in the container. 
 
 
 ## Logging

pages/docs/user-docs/docs-bind-paths.md

@@ -38,12 +38,16 @@ The system administrator has the ability to define what bind points will be incl
 
 
 #### User defined bind points
-If the system administrator has enabled user control of binds (via `user bind control = yes` in `/etc/singularity/singularity.conf`), you will be able to request your own bind points within your container processes. The most typical example of this is the `--bind` option and here is an example binding `/tmp` to `/scratch` (which again does not exist within the container):
+If the system administrator has enabled user control of binds (via `user bind control = yes` in `/etc/singularity/singularity.conf`), you will be able to request your own bind points within your container. 
+
+Further, if the administrator has enabled the use of file system overlay (via `enable overlay = yes` in `/etc/singularity/singularity.conf`), you can bind host system directories to directories that do not exist within the container.  Singularity will dynamically create the necessary bind points in your container on demand.  This feature may not be supported on older host systems.
+
+Here's an example of using the `--bind` option and binding `/tmp` to `/scratch` (which may not already exist within the container if file system overlay is enabled):
 
 ```bash
 $ singularity shell -B /tmp:/scratch /tmp/Centos7-ompi.img 
-WARNING: Skipping user bind, non existant bind point (directory) in container: '/scratch'
 Singularity: Invoking an interactive shell within container...
 
-Singularity.Centos7-ompi.img> 
+Singularity.Centos7-ompi.img> ls /scratch
+ssh-7vywtVeOez  systemd-private-cd84c81dda754fe4a7a593647d5a5765-ntpd.service-12nMO4
 ```