Fix doc nits

Many tiny changes to stdlib doc comments to make them consistent (for example
"Returns foo", rather than "Return foo", per RFC1574), adding missing periods, paragraph
breaks, backticks for monospace style, and other minor nits.

https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md#appendix-a-full-conventions-text

library/std/src/path.rs

@@ -1797,7 +1797,7 @@ impl<T: ?Sized + AsRef<OsStr>> From<&T> for PathBuf {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl From<OsString> for PathBuf {
-    /// Converts an [`OsString`] into a [`PathBuf`]
+    /// Converts an [`OsString`] into a [`PathBuf`].
     ///
     /// This conversion does not allocate or copy memory.
     #[inline]