Fixes the following warning when building for host
```
[...]/pico-sdk/src/common/pico_time/time.c: In function 'alarm_pool_dump_key':
[...]/pico-sdk/src/common/pico_time/time.c:282:15: warning: format '%ld' expects argument of type 'long int', but argument 2 has type 'uint64_t' {aka 'long long unsigned int'} [-Wformat=]
printf("%ld", to_us_since_boot(get_entry(pool, id)->target));
~~^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
%I64d
```
- Add recursive_mutex
- Make all locking primitives and sleep use common overridable wait/notify support to allow RTOS
implementations to replace WFE/SEV with something more appropriate
- Add busy_wait_ms
* Add documentation for gpio_irq_callback_t and rtc_callback_t (fixes#175)
* Hook up pico_binary_info and pico_bootsel_via_double_reset to the Doxygen index
* Add link to new Raspberry Pi Pico FAQ
* Consistently add parentheses for \sa or \see links to function-names
* Use consistent capitalisation for 'GitHub'
* Small typos and markup errors
In C, func() is a function taking an unspecified number of arguments,
vs func(void) a function taking no arguments. In C++ both forms indicate
"no arguments."
Update these headers to use the (void) form, which is correct in both
languages and avoids complaints when -Wstrict-prototypes is specified.
