Polish std::path::absolute documentation.

These changes bring it closer to other standard library documentation
and, in particular, `std::fs::canonicalize`, which it will often be
compared with.

* Add `# Platform-specific behavior` section, with content moved from
  Examples section.
* Create `# Errors` section.
* Phrase error description to allow future platforms to have new
  syntactic errors, rather than only emptiness.
* Add missing commas.
* Indent example code 4 spaces.

library/std/src/fs.rs

@@ -2302,7 +2302,7 @@ pub fn read_link<P: AsRef<Path>>(path: P) -> io::Result<PathBuf> {
 ///
 /// This function currently corresponds to the `realpath` function on Unix
 /// and the `CreateFile` and `GetFinalPathNameByHandle` functions on Windows.
-/// Note that, this [may change in the future][changes].
+/// Note that this [may change in the future][changes].
 ///
 /// On Windows, this converts the path to use [extended length path][path]
 /// syntax, which allows your program to use longer path names, but means you

library/std/src/path.rs

@@ -3345,65 +3345,76 @@ impl Error for StripPrefixError {
 /// Makes the path absolute without accessing the filesystem.
 ///
 /// If the path is relative, the current directory is used as the base directory.
-/// All intermediate components will be resolved according to platforms-specific
-/// rules but unlike [`canonicalize`][crate::fs::canonicalize] this does not
+/// All intermediate components will be resolved according to platform-specific
+/// rules, but unlike [`canonicalize`][crate::fs::canonicalize], this does not
 /// resolve symlinks and may succeed even if the path does not exist.
 ///
 /// If the `path` is empty or getting the
-/// [current directory][crate::env::current_dir] fails then an error will be
+/// [current directory][crate::env::current_dir] fails, then an error will be
 /// returned.
 ///
+/// # Platform-specific behavior
+///
+/// On POSIX platforms, the path is resolved using [POSIX semantics][posix-semantics],
+/// except that it stops short of resolving symlinks. This means it will keep `..`
+/// components and trailing slashes.
+///
+/// On Windows, for verbatim paths, this will simply return the path as given. For other
+/// paths, this is currently equivalent to calling
+/// [`GetFullPathNameW`][windows-path].
+///
+/// Note that these [may change in the future][changes].
+///
+/// # Errors
+///
+/// This function may return an error in the following situations:
+///
+/// * If `path` is syntactically invalid; in particular, if it is empty.
+/// * If getting the [current directory][crate::env::current_dir] fails.
+///
 /// # Examples
 ///
 /// ## POSIX paths
 ///
 /// ```
 /// # #[cfg(unix)]
 /// fn main() -> std::io::Result<()> {
-///   use std::path::{self, Path};
+///     use std::path::{self, Path};
 ///
-///   // Relative to absolute
-///   let absolute = path::absolute("foo/./bar")?;
-///   assert!(absolute.ends_with("foo/bar"));
+///     // Relative to absolute
+///     let absolute = path::absolute("foo/./bar")?;
+///     assert!(absolute.ends_with("foo/bar"));
 ///
-///   // Absolute to absolute
-///   let absolute = path::absolute("/foo//test/.././bar.rs")?;
-///   assert_eq!(absolute, Path::new("/foo/test/../bar.rs"));
-///   Ok(())
+///     // Absolute to absolute
+///     let absolute = path::absolute("/foo//test/.././bar.rs")?;
+///     assert_eq!(absolute, Path::new("/foo/test/../bar.rs"));
+///     Ok(())
 /// }
 /// # #[cfg(not(unix))]
 /// # fn main() {}
 /// ```
 ///
-/// The path is resolved using [POSIX semantics][posix-semantics] except that
-/// it stops short of resolving symlinks. This means it will keep `..`
-/// components and trailing slashes.
-///
 /// ## Windows paths
 ///
 /// ```
 /// # #[cfg(windows)]
 /// fn main() -> std::io::Result<()> {
-///   use std::path::{self, Path};
+///     use std::path::{self, Path};
 ///
-///   // Relative to absolute
-///   let absolute = path::absolute("foo/./bar")?;
-///   assert!(absolute.ends_with(r"foo\bar"));
+///     // Relative to absolute
+///     let absolute = path::absolute("foo/./bar")?;
+///     assert!(absolute.ends_with(r"foo\bar"));
 ///
-///   // Absolute to absolute
-///   let absolute = path::absolute(r"C:\foo//test\..\./bar.rs")?;
+///     // Absolute to absolute
+///     let absolute = path::absolute(r"C:\foo//test\..\./bar.rs")?;
 ///
-///   assert_eq!(absolute, Path::new(r"C:\foo\bar.rs"));
-///   Ok(())
+///     assert_eq!(absolute, Path::new(r"C:\foo\bar.rs"));
+///     Ok(())
 /// }
 /// # #[cfg(not(windows))]
 /// # fn main() {}
 /// ```
 ///
-/// For verbatim paths this will simply return the path as given. For other
-/// paths this is currently equivalent to calling
-/// [`GetFullPathNameW`][windows-path].
-///
 /// Note that this [may change in the future][changes].
 ///
 /// [changes]: io#platform-specific-behavior