Mention `core.excludesFile` in `git clean`'s documentation

[dscho]

"man git-clean" explicitly mentions two locations related to cleaning:

  -x
        Don't use the standard ignore rules read from .gitignore
        (per directory) and $GIT_DIR/info/exclude, ...

without additionally explicitly mentioning core.excludesFile. if the
man page simply said something like, "using the standard ignore
processing" and left it at that, it would be fine, but to list two of
the locations without the third is potentially confusing.

See https://public-inbox.org/git/nycvar.QRO.7.76.6.1902241515120.45@tvgsbejvaqbjf.bet/

[Denton-L]

This was closed by b582c16 (Merge branch 'dl/ignore-docs', 2019-04-10).

[dscho]

@Denton-L excellent, thanks!