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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

Documentation/git-init.txt

@@ -130,7 +130,12 @@ The template directory will be one of the following (in order):
  - the default template directory: `/usr/share/git-core/templates`.
 
 The default template directory includes some directory structure, suggested
-"exclude patterns" (see linkgit:gitignore[5]), and sample hook files (see linkgit:githooks[5]).
+"exclude patterns" (see linkgit:gitignore[5]), and sample hook files.
+
+The sample hooks are all disabled by default, To enable one of the
+sample hooks rename it by removing its `.sample` suffix.
+
+See linkgit:githooks[5] for more general info on hook execution.
 
 EXAMPLES
 --------

Documentation/githooks.txt

@@ -13,18 +13,25 @@ $GIT_DIR/hooks/*
 DESCRIPTION
 -----------
 
-Hooks are little scripts you can place in `$GIT_DIR/hooks`
-directory to trigger action at certain points.  When
-'git init' is run, a handful of example hooks are copied into the
-`hooks` directory of the new repository, but by default they are
-all disabled.  To enable a hook, rename it by removing its `.sample`
-suffix.
-
-NOTE: It is also a requirement for a given hook to be executable.
-However - in a freshly initialized repository - the `.sample` files are
-executable by default.
-
-This document describes the currently defined hooks.
+Hooks are programs you can place in the `$GIT_DIR/hooks` directory to
+trigger actions at certain points in git's execution. Hooks that don't
+have the executable bit set are ignored.
+
+Before Git invokes a hook, it changes its working directory to either
+the root of the working tree in a non-bare repository, or to the
+$GIT_DIR in a bare repository.
+
+Hooks can get their arguments via the environment, command-line
+arguments, and stdin. See the documentation for each hook below for
+details.
+
+'git init' may copy hooks to the new repository, depending on its
+configuration. See the "TEMPLATE DIRECTORY" section in
+linkgit:git-init[1] for details. When the rest of this document refers
+to "default hooks" it's talking about the default template shipped
+with Git.
+
+The currently supported hooks are described below.
 
 HOOKS
 -----