btrfs-progs: docs: update btrfs-convert manual page

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

Documentation/btrfs-convert.asciidoc

@@ -3,7 +3,7 @@ btrfs-convert(8)
 
 NAME
 ----
-btrfs-convert - convert from ext2/3/4 filesystem to btrfs
+btrfs-convert - convert from ext2/3/4 filesystem to btrfs in-place
 
 SYNOPSIS
 --------
@@ -28,6 +28,11 @@ steps to transform the btrfs filesystem to a more compact layout. The
 conversion inherits the original data block fragmentation and the metadata
 blocks are bound to the original free space layout.
 
+Due to different constraints, it's possible to convert only filesystem that
+have supported data block size (ie. the same that would be valid for
+'mkfs.btrfs'). This is typically the system page size (4KiB on x86_64
+machines).
+
 **REMOVE THE ORIGINAL FILESYSTEM METADATA**
 
 By removing the 'ext2_saved' subvolume, all metadata of the original filesystem
@@ -36,7 +41,7 @@ will be removed:
    # btrfs subvolume delete /mnt/ext2_saved
 
 At this point it's not possible to do rollback. The filesystem is usable but may
-be impacted by the fragmentation.
+be impacted by the fragmentation inherited from the original filesystem.
 
 **MAKE FILE DATA MORE CONTIGUOUS**
 
@@ -45,8 +50,8 @@ filesystem. This will attempt to make file extents more contiguous.
 
    # btrfs filesystem defrag -v -r -f -t 32M /mnt/btrfs
 
-Verbose recursive defragmentation ('-v', '-r'), flush data per-file ('-f') with target
+Verbose recursive defragmentation ('-v', '-r'), flush data per-file ('-f') with
-extent size 32M ('-t').
+target extent size 32MiB ('-t').
 
 **ATTEMPT TO MAKE BTRFS METADATA MORE COMPACT**
 
@@ -54,18 +59,19 @@ Optional but recommended step.
 
 The metadata block groups after conversion may be smaller than the default size
 (256MiB or 1GiB). Running a balance will attempt to merge the block groups.
-This depends on the free space layout (and fragmentation) and may fail. This is
+This depends on the free space layout (and fragmentation) and may fail due to
-a soft error leaving the filesystem usable but the block group layout may
+lack of enough work space. This is a soft error leaving the filesystem usable
-remain unchanged.
+but the block group layout may remain unchanged.
 
-Note that balance operation takes a lot of time.
+Note that balance operation takes a lot of time, please see also
+`btrfs-balance`(8).
 
    # btrfs balance start -m /mnt/btrfs
 
 OPTIONS
 -------
 -d|--no-datasum::
-disable data checksum calculations and set NODATASUM file flag, this can speed
+disable data checksum calculations and set the NODATASUM file flag, this can speed
 up the conversion
 -i|--no-xattr::
 ignore xattrs and ACLs of files
@@ -84,17 +90,20 @@ set filesystem label during conversion
 -L|--copy-label::
 use label from the converted filesystem
 -O|--features <feature1>[,<feature2>...]::
-A list of filesystem features turned on at btrfs-convert time. Not all features
+A list of filesystem features turned on at conversion time. Not all features
 are supported by old kernels. To disable a feature, prefix it with '^'.
+Description of the features is in section 'FILESYSTEM FEATURES' of
+`mkfs.btrfs`(8).
 +
 To see all available features that btrfs-convert supports run:
 +
 +btrfs-convert -O list-all+
 +
 -p|--progress::
-show progress of conversion, on by default
+show progress of conversion (a heartbeat indicator and number of inodes
+processed), on by default
 --no-progress::
-disable detailed progress and show only the main phases of conversion
+disable progress and show only the main phases of conversion
 
 EXIT STATUS
 -----------