s3:docs: add some advice for usage of strict allocate

docs-xml/smbdotconf/tuning/strictallocate.xml

@@ -10,14 +10,26 @@
     of actually forcing the disk system to allocate real storage blocks
     when a file is created or extended to be a given size. In UNIX
     terminology this means that Samba will stop creating sparse files.
-    This can be slow on some systems.</para>
+    This can be slow on some systems. When you work with large files like
+    >100MB or so you may even run into problems with clients running into
+    timeouts.</para>
 
-    <para>When strict allocate is <constant>no</constant> the server does sparse
-    disk block allocation when a file is extended.</para>
+    <para>When you have an extent based filesystem it's likely that we can make
+    use of unwritten extents which allows Samba to allocate even large ammounts
+    of space very fast and you will not see any timeout problems caused by
+    strict allocate. With strict allocate in use you will also get much better
+    out of quota messages in case you use quotas. Another advantage of
+    activating this setting is that it will help to reduce file
+    fragmentation.</para>
+
+    <para>To give you an idea on which filesystems this setting might currently
+    be a good option for you: XFS, ext4, btrfs, ocfs2 on Linux and JFS2 on
+    AIX support unwritten extents. On Filesystems that do not support it,
+    preallocation is probably an expensive operation where you will see reduced
+    performance and risk to let clients run into timeouts when creating large
+    files. Examples are ext3, ZFS, HFS+ and most others, so be aware if you
+    activate this setting on those filesystems.</para>
 
-    <para>Setting this to <constant>yes</constant> can help Samba return
-    out of quota messages on systems that are restricting the disk quota
-    of users.</para>
 </description>
 
 <value type="default">no</value>