runtime(doc): mention how to disable folding in diff mode (#13242)

Signed-off-by: Christian Brabandt <[email protected]>

Author: dundargoc

runtime/doc/options.txt

@@ -2839,7 +2839,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 				When omitted a context of six lines is used.
 				When using zero the context is actually one,
 				since folds require a line in between, also
-				for a deleted line.
+				for a deleted line. Set it to a very large
+				value (999999) to disable folding completely.
 				See |fold-diff|.
 
 		iblank		Ignore changes where lines are all blank.  Adds