Document the heuristics IsTerminal uses on Windows

Author: joshtriplett

library/std/src/io/stdio.rs

@@ -1054,6 +1054,10 @@ pub trait IsTerminal: crate::sealed::Sealed {
     /// On platforms where Rust does not know how to detect a terminal yet, this will return
     /// `false`. This will also return `false` if an unexpected error occurred, such as from
     /// passing an invalid file descriptor.
+    ///
+    /// 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.
     fn is_terminal(&self) -> bool;
 }