doc: Recommend running 'guix git authenticate' when cloning the repo.

* doc/contributing.texi (Building from Git): Adjust instruction and
recommend 'guix git authenticate'.

doc/contributing.texi

@@ -42,30 +42,27 @@ git clone https://git.savannah.gnu.org/git/guix.git
 
 @cindex authentication, of a Guix checkout
 How do you ensure that you obtained a genuine copy of the repository?
-Guix itself provides a tool to @dfn{authenticate} your checkout, but you
-must first make sure this tool is genuine in order to ``bootstrap'' the
-trust chain.  To do that, run:
+To do that, run @command{guix git authenticate}, passing if the commit
+and OpenPGP fingerprint of the @dfn{channel introduction}
+(@pxref{Invoking guix git authenticate}):
 
-@c XXX: Adjust instructions when there's a known tag to start from.
+@c The commit and fingerprint below must match those of the channel
+@c introduction in '%default-channels'.
 @example
-git verify-commit `git log --format=%H build-aux/git-authenticate.scm`
-@end example
-
-The output must look something like:
-
-@example
-gpg: Signature made Fri 27 Dec 2019 01:27:41 PM CET
-gpg:                using RSA key 3CE464558A84FDC69DB40CFB090B11993D9AEBB5
-@dots{}
-gpg: Signature made Fri 27 Dec 2019 01:25:22 PM CET
-gpg:                using RSA key 3CE464558A84FDC69DB40CFB090B11993D9AEBB5
-@dots{}
+guix git authenticate 9edb3f66fd807b096b48283debdcddccfea34bad \
+  "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA"
 @end example
 
 @noindent
-... meaning that changes to this file are all signed with key
-@code{3CE464558A84FDC69DB40CFB090B11993D9AEBB5} (you may need to fetch
-this key from a key server, if you have not done it yet).
+This command completes with exit code zero on success; it prints an
+error message and exits with a non-zero code otherwise.
+
+As you can see, there is a chicken-and-egg problem: you first need to
+have Guix installed.  Typically you would install Guix System
+(@pxref{System Installation}) or Guix on top of another distro
+(@pxref{Binary Installation}); in either case, you would verify the
+OpenPGP signature on the installation medium.  This ``bootstraps'' the
+trust chain.
 
 The easiest way to set up a development environment for Guix is, of
 course, by using Guix!  The following command starts a new shell where