patch 8.0.0179: cannot have a local value for 'formatprg'

Problem:    'formatprg' is a global option but the value may depend on the
            type of buffer. (Sung Pae)
Solution:   Make 'formatprg' global-local. (closes #1380)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 3d8e053..88dca60 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -3417,7 +3417,7 @@
 
 						*'formatprg'* *'fp'*
 'formatprg' 'fp'	string (default "")
-			global
+			global or local to buffer |global-local|
 			{not in Vi}
 	The name of an external program that will be used to format the lines
 	selected with the |gq| operator.  The program must take the input on