Update documentation for std::process::Command's new method

In the current documentation, it's not specified that when creating
a Command, the .exe extension can be omitted for Windows executables.
However, for other types of executable files like .bat or .cmd,
the complete filename including the extension must be provided.

I encountered it by noticing that `Command::new("wt").spawn().unwrap()`
succeeds on my machine while `Command::new("code").spawn().unwrap()`
panics. Turns out VS Code's entrypoint is .cmd file.

`resolve_exe` method mentions this behaviour in a comment[1], but it
makes sense to mention it at more visible place.

I've added this clarification to the documentation, which should
make it more accurate and helpful for Rust developers
working on the Windows platform.

[1] https://github.com/rust-lang/rust/blob/e7fda447e7d05b6ca431fc8fe8f489b1fda810bc/library/std/src/sys/windows/process.rs#L425

Author: sanchopanca

library/std/src/process.rs

@@ -558,6 +558,14 @@ impl Command {
     /// but this has some implementation limitations on Windows
     /// (see issue #37519).
     ///
+    /// # Platform-specific behavior
+    ///
+    /// Note on Windows: For executable files with the .exe extension,
+    /// it can be omitted when specifying the program for this Command.
+    /// However, if the file has a different extension,
+    /// a filename including the extension needs to be provided,
+    /// otherwise the file won't be found.
+    ///
     /// # Examples
     ///
     /// Basic usage: