btrfs-progs: reformat global options and update docs

With the growing list of global option we need to print them somewhere
in the help text and document them.

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

Documentation/btrfs.rst

@@ -4,7 +4,7 @@ btrfs(8)
 SYNOPSIS
 --------
 
-**btrfs** <command> [<args>]
+**btrfs** [global] <group> [<group>...] <command> [<args>]
 
 DESCRIPTION
 -----------
@@ -44,6 +44,30 @@ All numbers will be formatted according to the rules of the `C` locale
 For an overview of a given command use **btrfs command --help**
 or **btrfs [command...] --help --full** to print all available options.
 
+There are global options that are passed between *btrfs* and the *group* name
+and affect behaviour not specific to the command, e.g. verbosity or the type
+of the output.
+
+--format <format>
+        if supported by the command, print subcommand output in that format (text, json)
+
+-v|--verbose
+        increase verbosity of the subcommand\n"
+
+-q|--quiet
+        print only errors\n"
+
+--log <level>
+        set log level (default, info, verbose, debug, quiet)\n"
+
+The remaining options are relevant only for the main tool:
+
+--help
+        print condensed help for all subcommands
+
+--version
+        print version string
+
 COMMANDS
 --------
 

btrfs.c

@@ -32,7 +32,21 @@
 #include "cmds/commands.h"
 
 static const char * const btrfs_cmd_group_usage[] = {
-	"btrfs [--help] [--version] [--format <format>] [-v|--verbose] [-q|--quiet] [--log=level] <group> [<group>...] <command> [<args>]",
+	/*
+	 * The main command group is the only one that takes options so this
+	 * needs the newlines and manual formatting.
+	 */
+	"btrfs [global] <group> [<group>...] <command> [<args>]\n"
+	"\n"
+	"Global options:\n"
+	"  --format <format> if supported, print subcommand output in that format (text, json)\n"
+	"  -v|--verbose      increase verbosity of the subcommand\n"
+	"  -q|--quiet        print only errors\n"
+	"  --log <level>     set log level (default, info, verbose, debug, quiet)\n"
+	"\n"
+	"Options for the main command only:\n"
+	"  --help            print condensed help for all subcommands\n"
+	"  --version         print version string",
 	NULL
 };