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

@@ -657,33 +657,35 @@ SWAPFILE SUPPORT
 The swapfile is supported since kernel 5.0. Use `swapon`(8) to activate the
 swapfile. There are some limitations of the implementation in btrfs and linux
 swap subsystem:
- 
+
 * filesystem - must be only single device
 * filesystem - must have only 'single' data profile
 * swapfile - the containing subvolume cannot be snapshotted
 * swapfile - must be preallocated
 * swapfile - must be nodatacow (ie. also nodatasum)
 * swapfile - must not be compressed
- 
+
 The limitations come namely from the COW-based design and mapping layer of
 blocks that allows the advanced features like relocation and multi-device
 filesystems. However, the swap subsystem expects simpler mapping and no
 background changes of the file blocks once they've been attached to swap.
- 
+
 With active swapfiles, the following whole-filesystem operations will skip
 swapfile extents or may fail:
- 
+
 * balance - block groups with swapfile extents are skipped and reported, the rest will be processed normally
 * resize grow - unaffected
 * resize shrink - works as long as the extents are outside of the shrunk range
 * device add - a new device does not interfere with existing swapfile and this operation will work, though no new swapfile can be activated afterwards
 * device delete - if the device has been added as above, it can be also deleted
 * device replace - ditto
- 
+
 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
+--------------------
 
 CHECKSUM ALGORITHMS
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