btrfs-progs: check, update documentation

Add recently added options to manpage, a few other rewordings.

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

Documentation/btrfs-check.txt

@@ -3,7 +3,7 @@ btrfs-check(8)
 
 NAME
 ----
-btrfs-check - check or repair a btrfs filesystem offline
+btrfs-check - check or repair an unmounted btrfs filesystem
 
 SYNOPSIS
 --------
@@ -11,11 +11,11 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-*btrfs check* is used to check or repair a btrfs filesystem offline.
+*btrfs check* is used to check or repair an unmounted btrfs filesystem.
 
-NOTE: Since btrfs is under heavy development especially the *btrfs check*
-command, it is *highly* recommended to read the following btrfs wiki before
-executing *btrfs check* with '--repair' option: +
+NOTE: Since btrfs is under development, the *btrfs check* capabilities are
+continuously enhanced. It's highly recommended to read the following btrfs
+wiki before executing *btrfs check* with '--repair' option: +
 https://btrfs.wiki.kernel.org/index.php/Btrfsck
 
 *btrfsck* is an alias of *btrfs check* command and is now deprecated.
@@ -23,17 +23,22 @@ https://btrfs.wiki.kernel.org/index.php/Btrfsck
 OPTIONS
 -------
 -s|--super <superblock>::
-use <superblock>th superblock copy.
+use <superblock>th superblock copy, valid values are 0 up to 2 if the
+respective superblock offset is within the fileystem
 --repair::
-try to repair the filesystem.
+try to repair the filesystem
 --init-csum-tree::
-create a new CRC tree.
+create a new CRC tree and recalculate all checksums
 --init-extent-tree::
-create a new extent tree.
+create a new extent tree
 --check-data-csum::
-verify checkums of data blocks.
+verify checkums of data blocks
+--qgroup-report::
+verify qgroup accounting and compare against filesystem accounting
 --subvol-extents <subvolid>::
-show extent state for a subvolume.
+show extent state for a subvolume
+--tree-root <bytenr>::
+use the given bytenr for the tree root
 
 EXIT STATUS
 -----------