Revise the eprint(ln)! feature.

* Factor out the nigh-identical bodies of `_print` and `_eprint` to a helper
   function `print_to` (I was sorely tempted to call it `_doprnt`).
 * Update the issue number for the unstable `eprint` feature.
 * Add entries to the "unstable book" for `eprint` and `eprint_internal`.
 * Style corrections to the documentation.

src/doc/unstable-book/src/SUMMARY.md

@@ -130,6 +130,8 @@
     - [derive_eq](library-features/derive-eq.md)
     - [discriminant_value](library-features/discriminant-value.md)
     - [error_type_id](library-features/error-type-id.md)
+    - [eprint](library-features/eprint.md)
+    - [eprint_internal](library-features/eprint-internal.md)
     - [exact_size_is_empty](library-features/exact-size-is-empty.md)
     - [fd](library-features/fd.md)
     - [fd_read](library-features/fd-read.md)

src/doc/unstable-book/src/library-features/eprint-internal.md

@@ -0,0 +1,5 @@
+# `eprint_internal`
+
+This feature is internal to the Rust compiler and is not intended for general use.
+
+------------------------

src/doc/unstable-book/src/library-features/eprint.md

@@ -0,0 +1,13 @@
+# `eprint`
+
+The tracking issue for this feature is: [#40528]
+
+[#40528]: https://github.com/rust-lang/rust/issues/40528
+
+------------------------
+
+This feature enables the `eprint!` and `eprintln!` global macros,
+which are just like `print!` and `println!`, respectively, except that
+they send output to the standard _error_ stream, rather than standard
+output.  (`panic!` messages have always been written to standard error.)
+