doc/release-notes: add for pretty printing improvements

Change-Id: I829581a3f5b8b742e6c866dcdbbc635f91afceb5

doc/manual/rl-next/pretty-printing.md

@@ -0,0 +1,58 @@
+---
+synopsis: "Eliminate some pretty-printing surprises"
+cls: [1616, 1617, 1618]
+prs: [11100]
+credits: [alois31, roberth]
+category: Improvements
+---
+
+Some inconsistent and surprising behaviours have been eliminated from the pretty-printing used by the REPL and `nix eval`:
+* Lists and attribute sets that contain only a single item without nested structures are no longer sometimes inappropriately indented in the REPL, depending on internal state of the evaluator.
+* Empty attribute sets and derivations are no longer shown as `«repeated»`, since they are always cheap to print.
+  This matches the existing behaviour of `nix-instantiate` on empty attribute sets.
+  Empty lists were never printed as `«repeated»` already.
+* The REPL by default does not print nested attribute sets and lists, and indicates elided items with an ellipsis.
+  Previously, the ellipsis was printed even when the structure was empty, so that such items do not in fact exist.
+  Since this behaviour was confusing, it does not happen any more.
+
+Before:
+```
+nix-repl> :p let x = 1 + 2; in [ [ x ] [ x ] ]
+[
+  [
+    3
+  ]
+  [ 3 ]
+]
+
+nix-repl> let inherit (import <nixpkgs> { }) hello; in [ hello hello ]
+[
+  «derivation /nix/store/fqs92lzychkm6p37j7fnj4d65nq9fzla-hello-2.12.1.drv»
+  «repeated»
+]
+
+nix-repl> let x = {}; in [ x ]
+[
+  { ... }
+]
+```
+
+After:
+```
+nix-repl> :p let x = 1 + 2; in [ [ x ] [ x ] ]
+[
+  [ 3 ]
+  [ 3 ]
+]
+
+nix-repl> let inherit (import <nixpkgs> { }) hello; in [ hello hello ]
+[
+  «derivation /nix/store/fqs92lzychkm6p37j7fnj4d65nq9fzla-hello-2.12.1.drv»
+  «derivation /nix/store/fqs92lzychkm6p37j7fnj4d65nq9fzla-hello-2.12.1.drv»
+]
+
+nix-repl> let x = {}; in [ x ]
+[
+  { }
+]
+```