Add NEWS.unknown file to correct versions of introduction

It is not intended for the human reader, but to give less wrong versions
of first appearance displayed by `describe-function` or
`describe-variable` that would otherwise be derived from a notice in a
more recent NEWS* file.

It is better to add items to this file than changing historical NEWS*
files when the actual version of introduction is not known.

* etc/NEWS.unknown: New file.  Add entry for `nthcdr`.

(cherry picked from commit 495bf22727a0e980c4677ad4acd229e125d69d03)

diff --git a/etc/NEWS.unknown b/etc/NEWS.unknown
new file mode 100644 (file)
index 0000000..4a6d68d
--- /dev/null
+++ b/etc/NEWS.unknown
@@ -0,0 +1,14 @@
+This file contains mentions of functions and variables whose
+version of introduction would otherwise be guessed incorrectly
+by M-x describe-function.
+
+Since much of early Emacs source history is lost, these versions are
+conservative estimates: the actual version of first appearance may very
+well be much earlier.
+
+* Changes in Emacs 13.8
+This may be the earliest surviving version with source code, although
+damaged.  See
+https://github.com/larsbrinkhoff/emacs-history/decuslib.com/decus/vax85b/gnuemax
+
+** 'nthcdr'