Improve PDF documentation margins

Improve PDF documentation margins

Set body indent to 0 to make use of the horizontal space better (and
some reviewers thought it was also more readable).

Add some left and right margin to the warning boxes, otherwise they
drift too far off the page in combination with the above change.

Author: Noboru Saito <noborusai@gmail.com>
Reviewed-by: Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>
Reviewed-by: Florents Tselai <florents.tselai@gmail.com>
Reviewed-by: Tatsuo Ishii <ishii@postgresql.org>
Discussion: https://www.postgresql.org/message-id/flat/CAAM3qnLyMUD79XF+SqAVwWCwURCF3hyuFY9Ki9Csbqs-zMwwnw@mail.gmail.com