Rollup merge of rust-lang#41870 - excaliburHisSheath:iss29367-windows-docs, r=frewsxcv

Improve docs in os::windows::ffi and os::windows::fs

Part of rust-lang#29367

This PR makes changes to the documentation in `os::windows::ffi` and `os::windows::fs` with the goal of fleshing them out and bringing them in line with Rust's quality standards.

r? @steveklabnik

Author: Mark-Simulacrum

src/libstd/sys/windows/ext/ffi.rs

@@ -26,8 +26,22 @@ pub trait OsStringExt {
     /// Creates an `OsString` from a potentially ill-formed UTF-16 slice of
     /// 16-bit code units.
     ///
-    /// This is lossless: calling `.encode_wide()` on the resulting string
+    /// This is lossless: calling [`encode_wide`] on the resulting string
     /// will always return the original code units.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    /// use std::os::windows::prelude::*;
+    ///
+    /// // UTF-16 encoding for "Unicode".
+    /// let source = [0x0055, 0x006E, 0x0069, 0x0063, 0x006F, 0x0064, 0x0065];
+    ///
+    /// let string = OsString::from_wide(&source[..]);
+    /// ```
+    ///
+    /// [`encode_wide`]: ./trait.OsStrExt.html#tymethod.encode_wide
     #[stable(feature = "rust1", since = "1.0.0")]
     fn from_wide(wide: &[u16]) -> Self;
 }
@@ -42,11 +56,29 @@ impl OsStringExt for OsString {
 /// Windows-specific extensions to `OsStr`.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait OsStrExt {
-    /// Re-encodes an `OsStr` as a wide character sequence,
-    /// i.e. potentially ill-formed UTF-16.
+    /// Re-encodes an `OsStr` as a wide character sequence, i.e. potentially
+    /// ill-formed UTF-16.
+    ///
+    /// This is lossless: calling [`OsString::from_wide`] and then
+    /// `encode_wide` on the result will yield the original code units.
+    /// Note that the encoding does not add a final null terminator.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::ffi::OsString;
+    /// use std::os::windows::prelude::*;
+    ///
+    /// // UTF-16 encoding for "Unicode".
+    /// let source = [0x0055, 0x006E, 0x0069, 0x0063, 0x006F, 0x0064, 0x0065];
+    ///
+    /// let string = OsString::from_wide(&source[..]);
+    ///
+    /// let result: Vec<u16> = string.encode_wide().collect();
+    /// assert_eq!(&source[..], &result[..]);
+    /// ```
     ///
-    /// This is lossless. Note that the encoding does not include a final
-    /// null.
+    /// [`OsString::from_wide`]: ./trait.OsStringExt.html#tymethod.from_wide
     #[stable(feature = "rust1", since = "1.0.0")]
     fn encode_wide(&self) -> EncodeWide;
 }