docs/filesystem: Rework state overlay and /opt messaging a bit

Tweak the changes in #1191 to more clearly lay out the tradeoffs of
state overlays. Contrast it with transient root (since that was one of
the primary motivations for it), and generalize the "alternatives" to be
about both approaches.

Author: jlebon

docs/src/filesystem.md

@@ -231,9 +231,13 @@ The semantics here are somewhat nuanced:
 - Changes persist across reboots by default
 - During updates, new files from the container image override any locally modified version
 
-A disadvantage of this approach is that there is no equivalent to this feature in the
-Docker/podman ecosystem, and while its semantics can often trivially enable many applications that
-install into `/opt`, it can still be prone to "state drift".
+The advantages are:
+- It makes it very easy to make compatible applications that install into `/opt`.
+- In contrast to transient root (above), a smaller surface of the filesystem is mutable.
+
+The disadvantages are:
+- There is no equivalent to this feature in the Docker/Podman ecosystem.
+- It allows for some temporary state drift until the next update.
 
 To enable this feature, instantiate the `[email protected]`
 unit template on the target path. For example, for `/opt`:
@@ -242,8 +246,10 @@ unit template on the target path. For example, for `/opt`:
 RUN systemctl enable [email protected]
 ```
 
-### Alternatives to state overlays
+## More generally dealing with `/opt`
 
-See the section on `/opt` in [Image building and configuration guidance](building/guidance.md).
-Essentially, add a redirect such as a symbolic link from `/opt/someapp/logs` or other
-directories that should be writable+persistent to `/var/someapp/logs` instead.
+Both transient root and state overlays above provide ways for packages
+that install in `/opt` to operate. However, for maximum immutability the
+best approach is simply to symlink just the parts of the `/opt` needed
+into `/var`. See the section on `/opt` in [Image building and configuration
+guidance](building/guidance.md) for a more concrete example.