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/os/windows/process.rs

@@ -109,7 +109,7 @@ impl IntoRawHandle for process::ChildStderr {
     }
 }
 
-/// Create a `ChildStdin` from the provided `OwnedHandle`.
+/// Creates a `ChildStdin` from the provided `OwnedHandle`.
 ///
 /// The provided handle must be asynchronous, as reading and
 /// writing from and to it is implemented using asynchronous APIs.
@@ -122,7 +122,7 @@ impl From<OwnedHandle> for process::ChildStdin {
     }
 }
 
-/// Create a `ChildStdout` from the provided `OwnedHandle`.
+/// Creates a `ChildStdout` from the provided `OwnedHandle`.
 ///
 /// The provided handle must be asynchronous, as reading and
 /// writing from and to it is implemented using asynchronous APIs.
@@ -135,7 +135,7 @@ impl From<OwnedHandle> for process::ChildStdout {
     }
 }
 
-/// Create a `ChildStderr` from the provided `OwnedHandle`.
+/// Creates a `ChildStderr` from the provided `OwnedHandle`.
 ///
 /// The provided handle must be asynchronous, as reading and
 /// writing from and to it is implemented using asynchronous APIs.