doc: Improve environment documentation further

Make various other updates suggested during review of the rST conversion.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index b3871b1..9592d19 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -16,6 +16,7 @@
    devicetree/index
    distro
    driver-model/index
+   environment
    global_data
    logging
    makefiles