add the possibility to document bind_interrupts structs

the `bind_interrupts` macro creates a `struct` for the interrupts. it
was so far not possible to document those (except for STM32) and there
was no generic documentation being generated/added either, thus the
`missing_docs` lint was triggered for consumers which enabled it.

with this change it is now possible to manually add a comment on the
`struct` being defined in the macro invocation.

to show that this works one RP example has been modified accordingly.

examples/rp/src/bin/adc.rs

@@ -12,9 +12,12 @@ use embassy_rp::gpio::Pull;
 use embassy_time::Timer;
 use {defmt_rtt as _, panic_probe as _};
 
-bind_interrupts!(struct Irqs {
-    ADC_IRQ_FIFO => InterruptHandler;
-});
+bind_interrupts!(
+    /// Binds the ADC interrupts.
+    struct Irqs {
+        ADC_IRQ_FIFO => InterruptHandler;
+    }
+);
 
 #[embassy_executor::main]
 async fn main(_spawner: Spawner) {