Add liter for gas device classes (#2661)

Author: abmantis

docs/core/entity/number.md

@@ -49,7 +49,7 @@ If specifying a device class, your number entity will need to also return the co
 | `NumberDeviceClass.ENERGY_DISTANCE` | kWh/100km, mi/kWh, km/kWh | Energy per distance, this device class should be used to represent energy consumption by distance, for example the amount of electric energy consumed by an electric car.
 | `NumberDeviceClass.ENERGY_STORAGE` | J, kJ, MJ, GJ, mWh, Wh, kWh, MWh, GWh, TWh, cal, kcal, Mcal, Gcal | Stored energy, this device class should be used to represent stored energy, for example the amount of electric energy currently stored in a battery or the capacity of a battery. Represents _power_ over _time_. Not to be confused with `power`.
 | `NumberDeviceClass.FREQUENCY` | Hz, kHz, MHz, GHz | Frequency
-| `NumberDeviceClass.GAS` | m³, ft³, CCF | Volume of gas. Gas consumption measured as energy in kWh instead of a volume should be classified as energy.
+| `NumberDeviceClass.GAS` | L, m³, ft³, CCF | Volume of gas. Gas consumption measured as energy in kWh instead of a volume should be classified as energy.
 | `NumberDeviceClass.HUMIDITY` | % | Relative humidity
 | `NumberDeviceClass.ILLUMINANCE` | lx | Light level
 | `NumberDeviceClass.IRRADIANCE` | W/m², BTU/(h⋅ft²) | Irradiance

docs/core/entity/sensor.md

@@ -52,7 +52,7 @@ If specifying a device class, your sensor entity will need to also return the co
 | `SensorDeviceClass.ENERGY_STORAGE` | J, kJ, MJ, GJ, mWh, Wh, kWh, MWh, GWh, TWh, cal, kcal, Mcal, Gcal | Stored energy, this device class should be used for sensors representing stored energy, for example the amount of electric energy currently stored in a battery or the capacity of a battery. Represents _power_ over _time_. Not to be confused with `power`.
 | `SensorDeviceClass.ENUM` | | The sensor has a limited set of (non-numeric) states. The `options` property must be set to a list of possible states when using this device class.
 | `SensorDeviceClass.FREQUENCY` | Hz, kHz, MHz, GHz | Frequency
-| `SensorDeviceClass.GAS` | m³, ft³, CCF | Volume of gas. Gas consumption measured as energy in kWh instead of a volume should be classified as energy.
+| `SensorDeviceClass.GAS` | L, m³, ft³, CCF | Volume of gas. Gas consumption measured as energy in kWh instead of a volume should be classified as energy.
 | `SensorDeviceClass.HUMIDITY` | % | Relative humidity
 | `SensorDeviceClass.ILLUMINANCE` | lx | Light level
 | `SensorDeviceClass.IRRADIANCE` | W/m², BTU/(h⋅ft²) | Irradiance