rust: clk: document overflow panics in Hertz constructors

`Hertz::from_khz()`, `from_mhz()` and `from_ghz()` multiply their
argument by 1_000, 1_000_000 and 1_000_000_000 respectively without
checking for overflow. When `CONFIG_RUST_OVERFLOW_CHECKS` is enabled,
each panics once its argument exceeds `c_ulong::MAX` divided by that
factor. None of the three documents this. The panic occurs only at
runtime, when the argument is not a constant expression.

Add the missing `# Panics` sections stating the bound for each unit.

Fixes: d01d702056 ("rust: clk: Add initial abstractions")
Signed-off-by: Georgios Androutsopoulos <georgeandrout13@gmail.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Brian Masney <bmasney@redhat.com>

rust/kernel/clk.rs

@@ -35,16 +35,31 @@ impl Hertz {
     const GHZ_TO_HZ: c_ulong = 1_000_000_000;
 
     /// Create a new instance from kilohertz (kHz)
+    ///
+    /// # Panics
+    ///
+    /// Panics if `CONFIG_RUST_OVERFLOW_CHECKS` is enabled and `khz` is greater
+    /// than `c_ulong::MAX / 1_000`.
     pub const fn from_khz(khz: c_ulong) -> Self {
         Self(khz * Self::KHZ_TO_HZ)
     }
 
     /// Create a new instance from megahertz (MHz)
+    ///
+    /// # Panics
+    ///
+    /// Panics if `CONFIG_RUST_OVERFLOW_CHECKS` is enabled and `mhz` is greater
+    /// than `c_ulong::MAX / 1_000_000`.
     pub const fn from_mhz(mhz: c_ulong) -> Self {
         Self(mhz * Self::MHZ_TO_HZ)
     }
 
     /// Create a new instance from gigahertz (GHz)
+    ///
+    /// # Panics
+    ///
+    /// Panics if `CONFIG_RUST_OVERFLOW_CHECKS` is enabled and `ghz` is greater
+    /// than `c_ulong::MAX / 1_000_000_000`.
     pub const fn from_ghz(ghz: c_ulong) -> Self {
         Self(ghz * Self::GHZ_TO_HZ)
     }