Document the portability of various tar formats better

[Bruno Haible]

Hi,

While running the (automake-provided) 'dist' target of GNU gettext, I
encountered this error:

tar: 
gettext-0.19.8.1.74-72e49-dirty/gettext-tools/gnulib-tests/test-term-ostream-xterm-linux-mandriva.out:
 file name is too long (max 99); not dumped
tar: 
gettext-0.19.8.1.74-72e49-dirty/gettext-runtime/intl-csharp/doc/GNU_Gettext_GettextResourceManager.html:
 file name is too long (max 99); not dumped

My findings [1] confirm the description of the 'tar-ustar' and 'tar-pax'
option. However, I found the text
  "... is believed to be old enough ..."
  "... very modern platforms"
a bit vague to base decisions on. Therefore here is a documentation patch
to document the actual findings.

[1] https://savannah.gnu.org/support/index.php?109437

0001-doc-Document-the-portability-of-various-tar-formats-.patch
Description: Text Data