doc: Document the installation of Go source code.

* doc/guix.texi (Build Systems): Document '#:install-source?' for the Go
build system.

doc/guix.texi

@@ -3592,6 +3592,11 @@ package source code's remote URI and filesystem hierarchy structure.  In
 some cases, you will need to unpack the package's source code to a
 different directory structure than the one indicated by the import path,
 and @code{#:unpack-path} should be used in such cases.
+
+Packages that provide Go libraries should be installed along with their
+source code.  The key @code{#:install-source?}, which defaults to
+@code{#t}, controls whether or not the source code is installed.  It can
+be set to @code{#f} for packages that only provide executable files.
 @end defvr
 @defvr {Scheme Variable} glib-or-gtk-build-system