bug#81557: misleading documentation of filename-length-max

Hi,

The documentation at [1] says that file names longer than 99 characters
"are generally considered not to be portable in tarballs. See the tar-v7 and
 tar-ustar options below."

But [2] documents that this is only true for tar-v7, and that for
tar-ustar filename-length-max=155 should be safe.

This matters because now that (since Automake 1.18) the tar-ustar format
is the default, for most packages there is no need to use
  filename-length-max=99
any more. They can use
  filename-length-max=151
instead.

Bruno

[1] https://www.gnu.org/software/automake/manual/html_node/List-of-Automake-options.html#index-filename_002dlength_002dmax_003d99
[2] https://www.gnu.org/software/automake/manual/html_node/List-of-Automake-options.html#tar_002dformats