[PATCH 1/3] rust: error: improve `Error::from_errno` documentation

[Miguel Ojeda <ojeda@kernel.org>]

This constructor is public since commit 5ed147473458 ("rust: error:
make conversion functions public"), and we will refer to it from the
documentation of `to_result` in a later commit.

Thus improve its documentation, including adding examples.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/error.rs | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index a41de293dcd1..c415c3d3a3b6 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -101,8 +101,23 @@ macro_rules! declare_err {
 impl Error {
     /// Creates an [`Error`] from a kernel error code.
     ///
-    /// It is a bug to pass an out-of-range `errno`. `EINVAL` would
-    /// be returned in such a case.
+    /// `errno` must be within error code range (i.e. `>= -MAX_ERRNO && < 0`).
+    ///
+    /// It is a bug to pass an out-of-range `errno`. [`code::EINVAL`] is returned in such a case.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Error::from_errno(-1), EPERM);
+    /// assert_eq!(Error::from_errno(-2), ENOENT);
+    /// ```
+    ///
+    /// The following calls are considered a bug:
+    ///
+    /// ```
+    /// assert_eq!(Error::from_errno(0), EINVAL);
+    /// assert_eq!(Error::from_errno(-1000000), EINVAL);
+    /// ```
     pub fn from_errno(errno: crate::ffi::c_int) -> Error {
         if let Some(error) = Self::try_from_errno(errno) {
             error
-- 
2.51.0