Add docstring for PS module

ctru-rs/src/services/ps.rs

@@ -1,7 +1,15 @@
+//! Process Services (PS) module. This is used for miscellaneous utility tasks, but
+//! is particularly important because it is used to generate random data, which
+//! is required for common things like [`HashMap`](std::collections::HashMap).
+//! See also <https://www.3dbrew.org/wiki/Process_Services>
+
+/// PS handle. This must not be dropped in order for random generation
+/// to work (in most cases, the lifetime of an application).
 #[non_exhaustive]
 pub struct Ps;
 
 impl Ps {
+    /// Initialize the PS module.
     pub fn init() -> crate::Result<Self> {
         let r = unsafe { ctru_sys::psInit() };
         if r < 0 {