Docstring of compare-buffer-substrings

[Philipp Stephani <p.stephani2@gmail.com>]

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Hi,

compare-buffer-substring allows all its arguments to be nil, but the
docstring doesn't say that. Attached is a patch that amends the docstring.

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From ab84e3ce5df8a648814e9b30fa10aa401e1feafb Mon Sep 17 00:00:00 2001
From: Philipp Stephani <phst@google.com>
Date: Sun, 2 Oct 2016 19:58:39 +0200
Subject: [PATCH] Document nil args of compare-buffer-substrings

* editfns.c (Fcompare_buffer_substrings): Document behavior when
the arguments are nil.
---
 src/editfns.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/editfns.c b/src/editfns.c
index 0bcc439..dcb1d8d 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -2936,6 +2936,10 @@ Return -N if first string is less after N-1 chars, +N if first string is
 greater after N-1 chars, or 0 if strings match.
 The first substring is in BUFFER1 from START1 to END1 and the second
 is in BUFFER2 from START2 to END2.
+All arguments may be nil.  If BUFFER1 or BUFFER2 is nil, the current
+buffer is used.  If START1 or START2 is nil, the value of `point-min'
+in the respective buffers is used.  If END1 or END2 is nil, the value
+of `point-max' in the respective buffers is used.
 The value of `case-fold-search' in the current buffer
 determines whether case is significant or ignored.  */)
   (Lisp_Object buffer1, Lisp_Object start1, Lisp_Object end1, Lisp_Object buffer2, Lisp_Object start2, Lisp_Object end2)
-- 
2.10.0