--one-file-system: add macOS apfs notes, fixes #4876

(cherry picked from commit b7adee65bd6301ee9f4a7c2cd0aab9d823b08123)

src/borg/archiver.py

@@ -3448,13 +3448,13 @@ class Archiver:
         The ``-x`` or ``--one-file-system`` option excludes directories, that are mountpoints (and everything in them).
         It detects mountpoints by comparing the device number from the output of ``stat()`` of the directory and its
         parent directory. Specifically, it excludes directories for which ``stat()`` reports a device number different
-        from the device number of their parent. Be aware that in Linux (and possibly elsewhere) there are directories
-        with device number different from their parent, which the kernel does not consider a mountpoint and also the
-        other way around. Examples are bind mounts (possibly same device number, but always a mountpoint) and ALL
-        subvolumes of a btrfs (different device number from parent but not necessarily a mountpoint). Therefore when
-        using ``--one-file-system``, one should make doubly sure that the backup works as intended especially when using
-        btrfs. This is even more important, if the btrfs layout was created by someone else, e.g. a distribution
-        installer.
+        from the device number of their parent.
+        In general: be aware that there are directories with device number different from their parent, which the kernel
+        does not consider a mountpoint and also the other way around.
+        Linux examples for this are bind mounts (possibly same device number, but always a mountpoint) and ALL
+        subvolumes of a btrfs (different device number from parent but not necessarily a mountpoint).
+        macOS examples are the apfs mounts of a typical macOS installation.
+        Therefore, when using ``--one-file-system``, you should double-check that the backup works as intended.
 
 
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