patch 8.0.1319: can't build GUI on MS-Windows

Problem:    Can't build GUI on MS-Windows.
Solution:   Don't define the balloon_split() function in a GUI-only build.
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 2ce6c48..d753924 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2708,13 +2708,16 @@
 
 		When showing a balloon is not possible nothing happens, no
 		error message.
-		{only available when compiled with the +balloon_eval feature}
+		{only available when compiled with the +balloon_eval or
+		+balloon_eval_term feature}
 
 balloon_split({msg})					*balloon_split()*
 		Split {msg} into lines to be displayed in a balloon.  The
 		splits are made for the current window size and optimize to
 		show debugger output.
 		Returns a |List| with the split lines.
+		{only available when compiled with the +balloon_eval_term
+		feature}
 
 							*browse()*
 browse({save}, {title}, {initdir}, {default})