Documentation: clarify whitespace rules for trailers

Documentation: clarify whitespace rules for trailers

Commit e4319562bc (trailer: be stricter in parsing separators, 2016-11-02)
restricted whitespaces allowed by `git interpret-trailers` in the "token"
part of the trailers it reads. This commit didn't update the related
documentation in Documentation/git-interpret-trailers.txt though.

Also commit 60ef86a162 (trailer: support values folded to multiple lines,
2016-10-21) updated the documentation, but didn't make it clear how many
whitespace characters are allowed at the beginning of new lines in folded
values.

Let's fix both of these issues by rewriting the paragraph describing
what whitespaces are allowed by `git interpret-trailers` in the trailers
it reads.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>