git.txt: HEAD is not that special

git.txt: HEAD is not that special

The introductory text in "git help git" that describes HEAD called
it "a special ref".  It is special compared to the more regular refs
like refs/heads/master and refs/tags/v1.0.0, but not that special,
unlike truly special ones like FETCH_HEAD.

Rewrite a few sentences to also introduce the distinction between a
regular ref that contain the object name and a symbolic ref that
contain the name of another ref.  Update the description of HEAD
that point at the current branch to use the more correct term, a
"symbolic ref".

This was found as part of auditing the documentation and in-code
comments for uses of "special ref" that refer merely a "pseudo ref".

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