docs/libs/Makefile.am: Fix path to core docs.

docs/libs/Makefile.am: Fix path to core docs.

Original commit message from CVS:
* docs/libs/Makefile.am:
Fix path to core docs.
* gst/gstbin.c: (gst_bin_get_by_interface),
(gst_bin_iterate_all_by_interface):
Refix docs by also renaming 'interface' to 'iface' in implementation.
* docs/gst/gstreamer-sections.txt:
* gst/gstcaps.c:
* gst/gstchildproxy.c: (gst_child_proxy_base_init):
* gst/gstchildproxy.h:
* gst/gstelementfactory.c:
* gst/gstpadtemplate.h:
* libs/gst/controller/gstcontroller.c:
(gst_controlled_property_new):
Document more.