doc terminal-compatibility: document Unicode characters used in output

Not sure if this will be useful but the fact that we use very
few Unicode characters, suggests that we are insecure about
this. Having some kind of central and explicit listing might help
future decision-making.  Obviously, completions and translations use
more characters, but those are not as central.

doc_src/terminal-compatibility.rst

@@ -285,3 +285,13 @@ Or add this to your ``config.fish``::
         commandline -f repaint
         functions --erase GNU-screen-workaround
     end
+
+Unicode Codepoints
+------------------
+
+By default, fish outputs the following non-ASCII characters::
+
+    × ► ¶ ⏎ • ● … μ – ’ ‘ “ ” ← → ↑ ↓
+
+as well as control pictures (U+2400 through U+241F),
+and locale-specific ones in :ref:`translated strings <variables-locale>`.