githooks.txt: improve the intro section

Change the documentation so that:

 * We don't talk about "little scripts". Hooks can be as big as you
   want, and don't have to be scripts, just call them "programs".

 * We note that we change the working directory before a hook is called,
   nothing documented this explicitly, but the current behavior is
   predictable. It helps a lot to know what directory these hooks will
   be executed from.

 * We don't make claims about the example hooks which may not be true
   depending on the configuration of 'init.templateDir'. Clarify that
   we're talking about the default settings of git-init in those cases,
   and move some of this documentation into git-init's documentation
   about the default templates.

 * We briefly note in the intro that hooks can get their arguments in
   various different ways, and that how exactly is described below for
   each hook.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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