git-clean.txt: clarify ignore pattern files

git-clean.txt: clarify ignore pattern files

In the git-clean documentation, -x and -e documented .gitignore,
$GIT_DIR/info/excludes but neglected to mention the file pointed to by
core.excludesFile.

Remove specific list of files and, instead, reference gitignore(5)
documentation so that information is consolidated and the git-clean
documentation is more precise.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Helped-by: Martin Ågren <martin.agren@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>