Add `cto` NFS mount option requirement

The `nocto` NFS mount option can result in stale dentry cache which
leads to odd behavior like missing refs, up to the directory attribute
cache, when the dentry cache gets invalidated.

doc/administration/nfs.md

@@ -189,6 +189,7 @@ Note there are several options that you should consider using:
 | `nofail` | Don't halt boot process waiting for this mount to become available
 | `lookupcache=positive` | Tells the NFS client to honor `positive` cache results but invalidates any `negative` cache results. Negative cache results cause problems with Git. Specifically, a `git push` can fail to register uniformly across all NFS clients. The negative cache causes the clients to 'remember' that the files did not exist previously.
 | `hard` | Instead of `soft`. [Further details](#soft-mount-option).
+| `cto` | `cto` is the default option, which you should use. Do not use `nocto`. [Further details](#nocto-mount-option).
 
 #### `soft` mount option
 
@@ -225,6 +226,25 @@ the mount point. Use `SIGKILL` (`kill -9`) to deal with hung processes.
 The `intr` option
 [stopped working in the 2.6 kernel](https://access.redhat.com/solutions/157873).
 
+#### `nocto` mount option
+
+Do not use `nocto`. Instead, use `cto`, which is the default.
+
+When using `nocto`, the dentry cache is always used, up to `acdirmax` seconds (attribute cache time) from the time it's created.
+
+This results in stale dentry cache issues with multiple clients, where each client can see a different (cached)
+version of a directory.
+
+From the [Linux man page](https://linux.die.net/man/5/nfs), the important parts:
+
+> If the nocto option is specified, the client uses a non-standard heuristic to determine when files on the server have changed.
+>
+> Using the nocto option may improve performance for read-only mounts, but should be used only if the data on the server changes only occasionally.
+
+We have noticed this behavior in an issue about [refs not found after a push](https://gitlab.com/gitlab-org/gitaly/-/issues/2589),
+where newly added loose refs can be seen as missing on a different client with a local dentry cache, as
+[described in this issue](https://gitlab.com/gitlab-org/gitlab/-/issues/326066#note_539436931).
+
 ### A single NFS mount
 
 It's recommended to nest all GitLab data directories within a mount, that allows automatic