DOC-ONLY: improve multi-arch documentation

Document CLIB_MARCH_FN and CLIB_MARCH_FN_SELECT

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I661ba2af79db8205d70fa87203d40a2e31b795e4
diff --git a/docs/gettingstarted/developers/multiarch/index.rst b/docs/gettingstarted/developers/multiarch/index.rst
index 48065b8..a1dfe98 100644
--- a/docs/gettingstarted/developers/multiarch/index.rst
+++ b/docs/gettingstarted/developers/multiarch/index.rst
@@ -9,3 +9,4 @@
    :maxdepth: 1
 
    nodefns
+   arbfns