git symbolic-ref: documentation fix

The old "git symbolic-ref" manpage seemed to imply in one place that
symlinks are still the default way to represent symbolic references
and in another that symlinks are deprecated.  Fix the text and shorten
the justification for the change of implementation.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-symbolic-ref.txt

@@ -43,12 +43,9 @@ In the past, `.git/HEAD` was a symbolic link pointing at
 `refs/heads/master`.  When we wanted to switch to another branch,
 we did `ln -sf refs/heads/newbranch .git/HEAD`, and when we wanted
 to find out which branch we are on, we did `readlink .git/HEAD`.
-This was fine, and internally that is what still happens by
-default, but on platforms that do not have working symlinks,
-or that do not have the `readlink(1)` command, this was a bit
-cumbersome.  On some platforms, `ln -sf` does not even work as
-advertised (horrors).  Therefore symbolic links are now deprecated
-and symbolic refs are used by default.
+But symbolic links are not entirely portable, so they are now
+deprecated and symbolic refs (as described above) are used by
+default.
 
 'git symbolic-ref' will exit with status 0 if the contents of the
 symbolic ref were printed correctly, with status 1 if the requested