Documentation: point to related commands from gitignore

A frequently asked question on #git is how to stop tracking a file
that is mistakenly tracked by git.  A frequently attempted strategy is
to add such files to .gitignore.

Thus one might imagine that the gitignore documentation could be a
good entry point for 'git rm' documentation.  Add some
cross-references in this vein.

While at it, move a reference to update-index --assume-unchanged from
the DESCRIPTION to lower down on the page.  This way, the methodical
reader can benefit from first learning what excludes files do, then
how they relate to other git facilities.

Based-on-patch-by: Sitaram Chamarty <sitaram@atc.tcs.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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