cli: allow setting log level with '-l' flag

doc/river.1.scd

@@ -5,7 +5,7 @@ river - dynamic tiling Wayland compositor
 
 # SYNOPSIS
 
-*river* [*-c* _shell_command_]
+*river* [*-c* _shell_command_] [*-l* _log_level_]
 
 # DESCRIPTION
 
@@ -20,6 +20,11 @@ bspwm based on wlroots and written in Zig.
 	*riverctl*(1) and autostart programs. Make sure the script is
 	executable.
 
+*-l* _log_level_
+	Set the log level of river to a value from 0 to 7 with 0 being the
+	least verbose and 7 being the most verbose. The default log level of
+	release-safe builds is 5.
+
 # CONFIGURATION
 
 View border width, gap sizes, and programs which should float are