git-remote.txt: avoid sounding as if loose refs are the only ones in the world

git-remote.txt: avoid sounding as if loose refs are the only ones in the world

It was correct to say "The file $GIT_DIR/refs/heads/master stores the
commit object name at the tip of the master branch" in the older days,
but not anymore, as refs can be packed into $GIT_DIR/packed-refs file.

Update the document to talk in terms of a more abstract concept "ref" and
"symbolic ref" where we are not describing the underlying implementation
detail.

This on purpose leaves two instances of $GIT_DIR/ in the git-remote
documentation; they do talk about $GIT_DIR/remotes/ and $GIT_DIR/branches/
file hierarchy that used to be the place to store configuration around
remotes before the configuration mechanism took them over.

Signed-off-by: Junio C Hamano <gitster@pobox.com>