patch 9.0.0916: getbufline() is inefficient for getting a single line

Problem:    getbufline() is inefficient for getting a single line.
Solution:   Add getbufoneline().
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 086081f..b6d98b9 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -935,6 +935,7 @@
 
 Working with text in another buffer:
 	getbufline()		get a list of lines from the specified buffer
+	getbufoneline()		get a one line from the specified buffer
 	setbufline()		replace a line in the specified buffer
 	appendbufline()		append a list of lines in the specified buffer
 	deletebufline()		delete lines from a specified buffer