doc: Expand on prepare-root

Per review.

Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

docs/src/filesystem.md

@@ -186,13 +186,15 @@ To do this, set the
 transient = true
 ```
 
-option in `prepare-root.conf`.  In particular this will allow software to
+option in `/usr/lib/ostree/prepare-root.conf`.  In particular this will allow software to
 write (transiently, i.e. until the next reboot) to all top-level directories,
 including `/usr` and `/opt`, with symlinks to `/var` for content that should
 persist.
 
 This can be combined with `etc.transient` as well (below).
 
+More on prepare-root: <https://ostreedev.github.io/ostree/man/ostree-prepare-root.html>
+
 ## Enabling transient etc
 
 The default (per above) is to have `/etc` persist. If however you do
@@ -205,10 +207,12 @@ the
 transient = true
 ```
 
-option in `prepare-root.conf`.
+option in `/usr/lib/ostree/prepare-root.conf`.
 
 This can be combined with `root.transient` as well (above).
 
+More on prepare-root: <https://ostreedev.github.io/ostree/man/ostree-prepare-root.html>
+
 ## Enabling state overlays
 
 This feature enables a writable overlay on top of `/opt` (or really, any