[PATCH docs v2 1/2] local-zfs: rephrase ARC limit

[Maximiliano Sandoval <m.sandoval@proxmox.com>]

We rephrase the current section putting more emphasis on the current
state of things given than Proxmox VE 8.1 was released over two years
ago.

We also document ZFS' default size. See [zfs-6a629f3] for more details.

This also frames the problem for pre-existing setups in its own
paragraph.

[zfs-6a629f3] https://github.com/openzfs/zfs/commit/6a629f32344468ae81b264055916641480cb438d

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
---
 local-zfs.adoc | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/local-zfs.adoc b/local-zfs.adoc
index 85fa092..b3a462d 100644
--- a/local-zfs.adoc
+++ b/local-zfs.adoc
@@ -586,10 +586,15 @@ configured for the root user.
 Limit ZFS Memory Usage
 ~~~~~~~~~~~~~~~~~~~~~~
 
-ZFS uses '50 %' of the host memory for the **A**daptive **R**eplacement
-**C**ache (ARC) by default. For new installations starting with {pve} 8.1, the
-ARC usage limit will be set to '10 %' of the installed physical memory, clamped
-to a maximum of +16 GiB+. This value is written to `/etc/modprobe.d/zfs.conf`.
+ZFS uses '10 %' of the host memory, clamped to a maximum of +16 GiB+, for the
+**A**daptive **R**eplacement **C**ache (ARC) by default. This value is written
+to `/etc/modprobe.d/zfs.conf` during installation.
+
+Before {pve} 8.1, the ARC usage limit was not set during installation and
+matched ZFS' default value which was either 62.5% starting on ZFS 2.3.0 or 50%
+for older versions. For existing installations that predate {pve} 8.1, manual
+steps would have to be performed in order to lower the usage limit as described
+below.
 
 Allocating enough memory for the ARC is crucial for IO performance, so reduce it
 with caution. As a general rule of thumb, allocate at least +2 GiB Base + 1
-- 
2.47.3