Add documentation for prompt_hostname

doc_src/cmds/prompt_hostname.rst

@@ -0,0 +1,27 @@
+.. _cmd-prompt_hostname:
+
+prompt_hostname - print the hostname, shortened for use in the prompt
+=====================================================================
+
+Synopsis
+--------
+
+::
+
+    function fish_prompt
+        echo -n (whoami)@(prompt_hostname) (prompt_pwd) '$ '
+    end
+
+Description
+-----------
+
+``prompt_hostname`` prints a shortened version the current hostname for use in the prompt. It will print just the first component of the hostname, everything up to the first dot.
+
+Examples
+--------
+
+::
+
+    # The machine's full hostname is foo.bar.com
+    >_ prompt_hostname
+    foo