Add more ways to create a PathBuf to docs

The best way to do this wasn't in the documentation, and the ways that
were there needed some extra text to elaborate.

Fixes #40159

Author: steveklabnik

src/libstd/path.rs

@@ -51,10 +51,17 @@
 //! ```
 //! use std::path::PathBuf;
 //!
+//! // This way works...
 //! let mut path = PathBuf::from("c:\\");
+//!
 //! path.push("windows");
 //! path.push("system32");
+//!
 //! path.set_extension("dll");
+//!
+//! // ... but push is best used if you don't know everything up
+//! // front. If you do, this way is better:
+//! let path: PathBuf = ["c:\\", "windows", "system32.dll"].iter().collect();
 //! ```
 //!
 //! [`Component`]: ../../std/path/enum.Component.html
@@ -63,6 +70,7 @@
 //! [`Path`]: ../../std/path/struct.Path.html
 //! [`push`]: ../../std/path/struct.PathBuf.html#method.push
 //! [`String`]: ../../std/string/struct.String.html
+//!
 //! [`str`]: ../../std/primitive.str.html
 //! [`OsString`]: ../../std/ffi/struct.OsString.html
 //! [`OsStr`]: ../../std/ffi/struct.OsStr.html
@@ -1036,14 +1044,40 @@ impl<'a> cmp::Ord for Components<'a> {
 ///
 /// # Examples
 ///
+/// You can use [`push`] to build up a `PathBuf` from
+/// components:
+///
 /// ```
 /// use std::path::PathBuf;
 ///
-/// let mut path = PathBuf::from("c:\\");
+/// let mut path = PathBuf::new();
+///
+/// path.push(r"C:\");
 /// path.push("windows");
 /// path.push("system32");
+///
 /// path.set_extension("dll");
 /// ```
+///
+/// However, [`push`] is best used for dynamic situations. This is a better way
+/// to do this when you know all of the components ahead of time:
+///
+/// ```
+/// use std::path::PathBuf;
+///
+/// let path: PathBuf = [r"C:\", "windows", "system32.dll"].iter().collect();
+/// ```
+///
+/// We can still do better than this! Since these are all strings, we can use
+/// `From::from`:
+///
+/// ```
+/// use std::path::PathBuf;
+///
+/// let path = PathBuf::from(r"C:\windows\system32.dll");
+/// ```
+///
+/// Which method works best depends on what kind of situation you're in.
 #[derive(Clone)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct PathBuf {