docs/plugins/: Added videoscale to docs.

docs/plugins/: Added videoscale to docs.

Original commit message from CVS:
* docs/plugins/Makefile.am:
* docs/plugins/gst-plugins-base-plugins-docs.sgml:
* docs/plugins/gst-plugins-base-plugins-sections.txt:
Added videoscale to docs.
* gst/videorate/gstvideorate.c: (gst_video_rate_flush_prev),
(gst_video_rate_swap_prev), (gst_video_rate_event),
(gst_video_rate_chain):
Fix typo in docs.
* gst/videoscale/gstvideoscale.c: (gst_video_scale_class_init),
(gst_video_scale_init), (gst_video_scale_prepare_size),
(gst_video_scale_set_caps), (gst_video_scale_get_unit_size),
(gst_video_scale_fixate_caps), (gst_video_scale_transform):
* gst/videoscale/gstvideoscale.h:
Added docs, examples.
Some code cleanups.
Post errors instead of g_warning.