doc: check for absence of multiple terms in each entry of desc list

doc: check for absence of multiple terms in each entry of desc list

For simplifying automated translation of the documentation, it is better to
only present one term in each entry of a description list of options. This
is because most of these terms can automatically be marked as
notranslatable.

Also, due to portability issues, the script generate-configlist.sh can no
longer insert newlines in the output. However, the result is that it no
longer correctly handles multiple terms in a single entry of definition
lists.

As a result, we now check that these entries do not exist in the
documentation.

Reviewed-by: Collin Funk <collin.funk1@gmail.com>
Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>