patch 8.1.0702: ":sign place" only uses the current buffer

Problem:    ":sign place" only uses the current buffer.
Solution:   List signs for all buffers when there is no buffer argument.
            Fix error message for invalid buffer name in sign_place().
            (Yegappan Lakshmanan, closes #3774)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 8bb025c..1278c97 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -7971,6 +7971,9 @@
 			name	name of the defined sign
 			priority	sign priority
 
+		The returned signs in a buffer are ordered by their line
+		number.
+
 		Returns an empty list on failure or if there are no placed
 		signs.