doc: Mention effect of 'autoreconf -fi' on INSTALL

doc: Mention effect of 'autoreconf -fi' on INSTALL

Several projects have a bootstrap script that invokes
'autoreconf -fi' as part of a fresh version control checkout,
in order to avoid storing common files in version control,
while also allowing contributors to rerun bootstrap to pick
up the benefits of any upgrade of one of the autotools.

However, the documentation did not make it obvious that such
a setup will overwrite any customizations to files like
INSTALL, if those files are stored in version control, when
automake still considers that file to be standard based on
AM_INIT_AUTOMAKE settings.  In such a case, a mere
'autoreconf -i' is good for the bootstrap script, while a
separate 'autoreconf -f' is good for picking up on an upgrade
of any autotools.

* bin/autoreconf.in (help): Mention standard files.
* doc/autoconf.texi (autoreconf Invocation): Add more text, including
warning that mixing --force and --install may undo customizations,
and that the set of files impacted is controlled by automake.
Reported by Emil Laine <laine.emil@gmail.com>
Signed-off-by: Eric Blake <eblake@redhat.com>