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 <[email protected]>

Author: gitster

Documentation/git.txt

@@ -1025,10 +1025,11 @@ When first created, objects are stored in individual files, but for
 efficiency may later be compressed together into "pack files".
 
 Named pointers called refs mark interesting points in history.  A ref
-may contain the SHA-1 name of an object or the name of another ref.  Refs
-with names beginning `ref/head/` contain the SHA-1 name of the most
+may contain the SHA-1 name of an object or the name of another ref (the
+latter is called a "symbolic ref").
+Refs with names beginning `refs/head/` contain the SHA-1 name of the most
 recent commit (or "head") of a branch under development.  SHA-1 names of
-tags of interest are stored under `ref/tags/`.  A special ref named
+tags of interest are stored under `refs/tags/`.  A symbolic ref named
 `HEAD` contains the name of the currently checked-out branch.
 
 The index file is initialized with a list of all paths and, for each