Rollup merge of rust-lang#45151 - Pirh:stdio_default_docs, r=frewsxcv

Document defaults for stdin, stdout, and stderr methods of Command

For rust-lang#29370

Author: kennytm

src/libstd/process.rs

@@ -552,6 +552,12 @@ impl Command {
 
     /// Configuration for the child process's standard input (stdin) handle.
     ///
+    /// Defaults to [`inherit`] when used with `spawn` or `status`, and
+    /// defaults to [`piped`] when used with `output`.
+    ///
+    /// [`inherit`]: struct.Stdio.html#method.inherit
+    /// [`piped`]: struct.Stdio.html#method.piped
+    ///
     /// # Examples
     ///
     /// Basic usage:
@@ -572,6 +578,12 @@ impl Command {
 
     /// Configuration for the child process's standard output (stdout) handle.
     ///
+    /// Defaults to [`inherit`] when used with `spawn` or `status`, and
+    /// defaults to [`piped`] when used with `output`.
+    ///
+    /// [`inherit`]: struct.Stdio.html#method.inherit
+    /// [`piped`]: struct.Stdio.html#method.piped
+    ///
     /// # Examples
     ///
     /// Basic usage:
@@ -592,6 +604,12 @@ impl Command {
 
     /// Configuration for the child process's standard error (stderr) handle.
     ///
+    /// Defaults to [`inherit`] when used with `spawn` or `status`, and
+    /// defaults to [`piped`] when used with `output`.
+    ///
+    /// [`inherit`]: struct.Stdio.html#method.inherit
+    /// [`piped`]: struct.Stdio.html#method.piped
+    ///
     /// # Examples
     ///
     /// Basic usage: