Add "Platform-specific behavior" heading and link to changes disclaimer

Author: joshtriplett

Diff for: library/std/src/io/stdio.rs

@@ -1055,9 +1055,14 @@ pub trait IsTerminal: crate::sealed::Sealed {
     /// `false`. This will also return `false` if an unexpected error occurred, such as from
     /// passing an invalid file descriptor.
     ///
+    /// # Platform-specific behavior
+    ///
     /// On Windows, in addition to detecting consoles, this currently uses some heuristics to
     /// detect older msys/cygwin/mingw pseudo-terminals based on device name: devices with names
     /// starting with `msys-` or `cygwin-` and ending in `-pty` will be considered terminals.
+    /// Note that this [may change in the future][changes].
+    ///
+    /// [changes]: io#platform-specific-behavior
     fn is_terminal(&self) -> bool;
 }