doc: Clarify that guix-daemon doesn't have to be launched from the checkout.

Suggested by Emmanuel Agullo.

* doc/contributing.texi (Running Guix Before It Is Installed): Move the
"./pre-inst-env guix-daemon" snippet at the bottom.  Make it clear that
it's necessary only when hacking on guix-daemon or when guix-daemon is
not already running.

doc/contributing.texi

@@ -160,13 +160,11 @@ with all the dependencies available (@pxref{Building from Git}), and then
 simply prefix each command with
 @command{./pre-inst-env} (the @file{pre-inst-env} script lives in the
 top build tree of Guix; it is generated by @command{./configure}).
-An example@footnote{The @option{-E} flag to
-@command{sudo} guarantees that @code{GUILE_LOAD_PATH} is correctly set
-such that @command{guix-daemon} and the tools it uses can find the Guile
-modules they need.}:
+As an example, here is how you would build the @code{hello} package as
+defined in your working tree (this assumes @command{guix-daemon} is
+already running on your system; it's OK if it's a different version):
 
 @example
-$ sudo -E ./pre-inst-env guix-daemon --build-users-group=guixbuild
 $ ./pre-inst-env guix build hello
 @end example
 
@@ -201,6 +199,17 @@ scheme@@(guile-user)> (length snakes)
 $1 = 361
 @end example
 
+If you are hacking on the daemon and its supporting code or if
+@command{guix-daemon} is not already running on your system, you can
+launch it straight from the build tree@footnote{The @option{-E} flag to
+@command{sudo} guarantees that @code{GUILE_LOAD_PATH} is correctly set
+such that @command{guix-daemon} and the tools it uses can find the Guile
+modules they need.}:
+
+@example
+$ sudo -E ./pre-inst-env guix-daemon --build-users-group=guixbuild
+@end example
+
 The @command{pre-inst-env} script sets up all the environment variables
 necessary to support this, including @env{PATH} and @env{GUILE_LOAD_PATH}.