Remove dunderscores from __fish_status_to_signal (#7597)

* Remove dunderscores from __fish_status_to_signal

* Document fish_status_to_signal

* CHANGELOG: Add fish_status_to_signal

* Add string join to fish_status_to_signal documentation example

doc_src/cmds/fish_status_to_signal.rst

@@ -0,0 +1,29 @@
+.. _cmd-fish_status_to_signal:
+
+fish_status_to_signal - Convert exit codes to human-friendly signals
+====================================================================
+
+Synopsis
+--------
+
+::
+
+    function fish_prompt
+        echo -n (fish_status_to_signal $pipestatus | string join '|') (prompt_pwd) '$ '
+    end
+
+Description
+-----------
+
+``fish_status_to_signal`` converts exit codes to their corresponding human-friendly signals if one exists.
+This is likely to be useful for prompts in conjunction with the ``$status`` and ``$pipestatus`` variables.
+
+Example
+-------
+
+::
+
+    >_ sleep 5
+    ^C⏎
+    >_ fish_status_to_signal $status
+    SIGINT