Rollup merge of #37473 - joshtriplett:doc-copyedit-write-writeln, r=alexcrichton

Rollup merge of #37473 - joshtriplett:doc-copyedit-write-writeln, r=alexcrichton

Copyediting on documentation for write! and writeln!

Fix various sentence fragments, missing articles, and other grammatical issues in the documentation for write! and writeln!.

Also fix the links (and link names) for common return types.

(Noticed when preparing https://github.com/rust-lang/rust/pull/37472 ; posted separately to avoid mixing the new documentation with copyedits to existing documentation.)