btrfs-progs: docs: how to use swapfile from fstab

This was asked on reddit, how to automatically mount a swapfile from
fstab. As this is not completely obvious, document it with an example.

Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/btrfs-man5.asciidoc

@@ -684,6 +684,8 @@ When there are no active swapfiles and a whole-filesystem exclusive operation
 is running (ie. balance, device delete, shrink), the swapfiles cannot be
 temporarily activated. The operation must finish first.
 
+To create and activate a swapfile run the following commands:
+
 --------------------
 # truncate -s 0 swapfile
 # chattr +C swapfile
@@ -693,6 +695,28 @@ temporarily activated. The operation must finish first.
 # swapon swapfile
 --------------------
 
+Please note that the UUID returned by the 'mkswap' utility identifies the swap
+"filesystem" and because it's stored in a file, it's not generally visible and
+usable as an identifier unlike if it was on a block device.
+
+The file will appear in '/proc/swaps':
+
+--------------------
+# cat /proc/swaps
+Filename          Type          Size           Used      Priority
+/path/swapfile    file          2097152        0         -2
+--------------------
+
+The swapfile can be created as one-time operation or, once properly created,
+activated on each boot by the 'swapon -a' command (usually started by the
+service manager). Add the following entry to '/etc/fstab', assuming the
+filesystem that provides the '/path' has been already mounted at this point.
+Additional mount options relevant for the swapfile can be set too (like
+priority, not the btrfs mount options).
+
+--------------------
+/path/swapfile        none        swap        defaults      0 0
+--------------------
 
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