doc: Document the location of the build directory.

Fixes <http://bugs.gnu.org/19771>.
Reported by Andrei Osipov <andrspv@gmail.com>.

* doc/guix.texi (Invoking guix-daemon): Document the build directory
  location.

doc/guix.texi

@@ -557,6 +557,19 @@ system directories.  By default, the latter contains @file{/dev} and
 a separate mount name space, its own PID name space, network name space,
 etc.  This helps achieve reproducible builds (@pxref{Features}).
 
+When the daemon performs a build on behalf of the user, it creates a
+build directory under @file{/tmp} or under the directory specified by
+its @code{TMPDIR} environment variable; this directory is shared with
+the container for the duration of the build.  Be aware that using a
+directory other than @file{/tmp} can affect build results---for example,
+with a longer directory name, a build process that uses Unix-domain
+sockets might hit the name length limitation for @code{sun_path}, which
+it would otherwise not hit.
+
+The build directory is automatically deleted upon completion, unless the
+build failed and the client specified @option{--keep-failed}
+(@pxref{Invoking guix build, @option{--keep-failed}}).
+
 The following command-line options are supported:
 
 @table @code