[mcp23016] Add interrupt_pin documentation (#6432)

Author: bdraco

src/content/docs/components/mcp230xx.mdx

@@ -91,6 +91,7 @@ has 16 GPIOs and can be configured the same way than the other variants.
 mcp23016:
   - id: 'mcp23016_hub'
     address: 0x20
+    interrupt_pin: GPIOXX
 
 # Individual outputs
 switch:
@@ -122,6 +123,11 @@ binary_sensor:
 - **id** (**Required**, [ID](/guides/configuration-types#id)): The id to use for this MCP23016 component.
 - **address** (*Optional*, int): The I²C address of the driver.
   Defaults to `0x20`.
+- **interrupt_pin** (*Optional*, [Pin Schema](/guides/configuration-types#pin-schema)): The pin connected to the
+  INT output of the MCP23016. When configured, the component becomes interrupt-driven instead of
+  polling — it only reads the chip when a pin actually changes state, significantly reducing I²C bus traffic
+  and CPU usage. The INT pin is active-low and open-drain, so an external or internal pull-up resistor is
+  required. Must be an internal GPIO pin (not another expander pin).
 
 ### Pin configuration variables