Rollup merge of rust-lang#63846 - DevQps:32626-document-time-system-c…

…alls, r=rkruppe

Added table containing the system calls used by Instant and SystemTime.

# Description
See rust-lang#32626 for a discussion on documenting system calls used by Instant and SystemTime.

## Changes
- Added a table containing the system calls used by each platform.

EDIT: If I can format this table better (due to the large links) please let me know.
I'd also be happy to learn a quick command to generate the docs on my host machine! Currently I am using: `python x.py doc --stage 0 src/libstd` but that gives me some `unrecognized intrinsic` errors. Advice is always welcome :)

closes rust-lang#32626

src/libstd/time.rs

@@ -59,6 +59,30 @@ pub use core::time::Duration;
 ///    println!("{}", now.elapsed().as_secs());
 /// }
 /// ```
+///
+/// # Underlying System calls
+/// Currently, the following system calls are being used to get the current time using `now()`:
+///
+/// |  Platform |               System call                                            |
+/// |:---------:|:--------------------------------------------------------------------:|
+/// | Cloud ABI | [clock_time_get (Monotonic Clock)]                                   |
+/// | SGX       | [`insecure_time` usercall]. More information on [timekeeping in SGX] |
+/// | UNIX      | [clock_time_get (Monotonic Clock)]                                   |
+/// | Darwin    | [mach_absolute_time]                                                 |
+/// | VXWorks   | [clock_gettime (Monotonic Clock)]                                    |
+/// | WASI      | [__wasi_clock_time_get (Monotonic Clock)]                            |
+/// | Windows   | [QueryPerformanceCounter]                                            |
+///
+/// [QueryPerformanceCounter]: https://docs.microsoft.com/en-us/windows/win32/api/profileapi/nf-profileapi-queryperformancecounter
+/// [`insecure_time` usercall]: https://edp.fortanix.com/docs/api/fortanix_sgx_abi/struct.Usercalls.html#method.insecure_time
+/// [timekeeping in SGX]: https://edp.fortanix.com/docs/concepts/rust-std/#codestdtimecode
+/// [__wasi_clock_time_get (Monotonic Clock)]: https://github.com/CraneStation/wasmtime/blob/master/docs/WASI-api.md#clock_time_get
+/// [clock_gettime (Monotonic Clock)]: https://linux.die.net/man/3/clock_gettime
+/// [mach_absolute_time]: https://developer.apple.com/library/archive/documentation/Darwin/Conceptual/KernelProgramming/services/services.html
+/// [clock_time_get (Monotonic Clock)]: https://github.com/NuxiNL/cloudabi/blob/master/cloudabi.txt
+///
+/// **Disclaimer:** These system calls might change over time.
+///
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[stable(feature = "time2", since = "1.8.0")]
 pub struct Instant(time::Instant);
@@ -114,6 +138,28 @@ pub struct Instant(time::Instant);
 ///    }
 /// }
 /// ```
+///
+/// # Underlying System calls
+/// Currently, the following system calls are being used to get the current time using `now()`:
+///
+/// |  Platform |               System call                                            |
+/// |:---------:|:--------------------------------------------------------------------:|
+/// | Cloud ABI | [clock_time_get (Realtime Clock)]                                    |
+/// | SGX       | [`insecure_time` usercall]. More information on [timekeeping in SGX] |
+/// | UNIX      | [clock_gettime (Realtime Clock)]                                     |
+/// | DARWIN    | [gettimeofday]                                                       |
+/// | VXWorks   | [clock_gettime (Realtime Clock)]                                     |
+/// | WASI      | [__wasi_clock_time_get (Realtime Clock)]                             |
+/// | Windows   | [GetSystemTimeAsFileTime]                                            |
+///
+/// [clock_time_get (Realtime Clock)]: https://github.com/NuxiNL/cloudabi/blob/master/cloudabi.txt
+/// [gettimeofday]: http://man7.org/linux/man-pages/man2/gettimeofday.2.html
+/// [clock_gettime (Realtime Clock)]: https://linux.die.net/man/3/clock_gettime
+/// [__wasi_clock_time_get (Realtime Clock)]: https://github.com/CraneStation/wasmtime/blob/master/docs/WASI-api.md#clock_time_get
+/// [GetSystemTimeAsFileTime]: https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-getsystemtimeasfiletime
+///
+/// **Disclaimer:** These system calls might change over time.
+///
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[stable(feature = "time2", since = "1.8.0")]
 pub struct SystemTime(time::SystemTime);