doc: Document the portability of various tar formats better

* doc/automake.texi (List of Automake options): Document the portability of
the tar-ustar and tar-pax options better.

1 files changed, 9 insertions, 6 deletions

diff --git a/doc/automake.texi b/doc/automake.texi
index 61c33a5a3..2df214a01 100644
--- a/doc/automake.texi
+++ b/doc/automake.texi
@@ -10332,20 +10332,23 @@ directories.  When using this format, consider using the
 @option{filename-length-max=99} option to catch file names too long.
 
 @option{tar-ustar} selects the ustar format defined by POSIX
-1003.1-1988.  This format is believed to be old enough to be portable.
+1003.1-1988.  This format is old enough to be portable:
+As of 2018, it is supported by the native @code{tar} command on
+GNU, FreeBSD, NetBSD, OpenBSD, AIX, HP-UX, Solaris, at least.
 It fully supports empty directories.  It can store file names with up
 to 256 characters, provided that the file name can be split at
 directory separator in two parts, first of them being at most 155
 bytes long.  So, in most cases the maximum file name length will be
-shorter than 256 characters.  However you may run against broken tar
-implementations that incorrectly handle file names longer than 99
-characters (please report them to @email{@value{PACKAGE_BUGREPORT}} so we
-can document this accurately).
+shorter than 256 characters.
 
 @option{tar-pax} selects the new pax interchange format defined by POSIX
 1003.1-2001.  It does not limit the length of file names.  However,
 this format is very young and should probably be restricted to
-packages that target only very modern platforms.  There are moves to
+packages that target only very modern platforms.
+As of 2018, this format is supported by the native @code{tar} command only
+on GNU, FreeBSD, OpenBSD system; it is not supported by the native
+@code{tar} command on NetBSD, AIX, HP-UX, Solaris.
+There are moves to
 change the pax format in an upward-compatible way, so this option may
 refer to a more recent version in the future.