Documentation/CodingGuidelines: spell out more shell guidelines

Documentation/CodingGuidelines: spell out more shell guidelines

In earlier days, "imitate the style in the neibouring code" was
sufficient to keep the coherent style, but over time some parts of
the codebase have drifted enough to make it ineffective.

Spell some of the guidelines out.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>