Say to use test -e in preference to test -a

[積丹尼 Dan Jacobson]

$ man bash #and
$ help test #say
      -a FILE        True if file exists.
      -e FILE        True if file exists.
OK, but add a note that it would be better to use -e,
as it is more portable.

Compare
$ test -a .
$ /usr/bin/test -a .