docs: establish one section per backend

docs/env_vars.md

@@ -2,12 +2,6 @@ wlroots reads these environment variables
 
 # wlroots specific
 
-* *WLR_DRM_DEVICES*: specifies the DRM devices (as a colon separated list)
-  instead of auto probing them. The first existing device in this list is
-  considered the primary DRM device.
-* *WLR_DRM_NO_ATOMIC*: set to 1 to use legacy DRM interface instead of atomic
-  mode setting
-* *WLR_LIBINPUT_NO_DEVICES*: set to 1 to not fail without any input devices
 * *WLR_BACKENDS*: comma-separated list of backends to use (available backends:
   libinput, drm, wayland, x11, headless, noop)
 * *WLR_NO_HARDWARE_CURSORS*: set to 1 to use software cursors instead of
@@ -16,16 +10,28 @@ wlroots reads these environment variables
   logind/systemd, direct)
 * *WLR_DIRECT_TTY*: specifies the tty to be used (instead of using /dev/tty)
 
-# Headless backend
+## DRM backend
+
+* *WLR_DRM_DEVICES*: specifies the DRM devices (as a colon separated list)
+  instead of auto probing them. The first existing device in this list is
+  considered the primary DRM device.
+* *WLR_DRM_NO_ATOMIC*: set to 1 to use legacy DRM interface instead of atomic
+  mode setting
+
+## Headless backend
 
 * *WLR_HEADLESS_OUTPUTS*: when using the headless backend specifies the number
   of outputs
 
-# Wayland backend
+## libinput backend
+
+* *WLR_LIBINPUT_NO_DEVICES*: set to 1 to not fail without any input devices
+
+## Wayland backend
 
 * *WLR_WL_OUTPUTS*: when using the wayland backend specifies the number of outputs
 
-# X11 backend
+## X11 backend
 
 * *WLR_X11_OUTPUTS*: when using the X11 backend specifies the number of outputs