patch 9.1.0099: Not able to use diff() with 'diffexpr'

Problem:  Not able to use diff() with 'diffexpr'
          (rickhowe, after v9.1.0096)
Solution: Use a default context length of 0, update diff() help text,
          add a test for using diff() with 'diffexpr'
          (Yegappan Lakshmanan)

closes: #14013

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/diff.txt b/runtime/doc/diff.txt
index 05dd4a6..e3abbde 100644
--- a/runtime/doc/diff.txt
+++ b/runtime/doc/diff.txt
@@ -510,9 +510,14 @@
    [{'from_idx': 1, 'from_count': 2, 'to_idx': 1, 'to_count': 0}]
 
   " disjointed changes
-  :echo diff(['ab', 'def', 'ghi', 'jkl'], ['abc', 'def', 'ghi', 'jk'], {'output': 'indices'})
+  :echo diff(['ab', 'def', 'ghi', 'jkl'], ['abc', 'def', 'ghi', 'jk'], {'output': 'indices', 'context': 0})
    [{'from_idx': 0, 'from_count': 1, 'to_idx': 0, 'to_count': 1},
     {'from_idx': 3, 'from_count': 1, 'to_idx': 3, 'to_count': 1}]
+
+  " disjointed changes with context length 1
+  :echo diff(['ab', 'def', 'ghi', 'jkl'], ['abc', 'def', 'ghi', 'jk'], {'output': 'indices', 'context': 1})
+   [{'from_idx': 0, 'from_count': 4, 'to_idx': 0, 'to_count': 4}]
+
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  vim:tw=78:ts=8:noet:ft=help:norl: