snippet: Add some more documentation

This adds a documentation section for CoglSnippet which gives an
overview of how to use them. It also fixes some syntax errors in the
existing documentation and adds the missing pipeline functions for
adding snippets to the documentation sections file.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

cogl/cogl-pipeline-state.h

@@ -943,11 +943,10 @@ cogl_pipeline_set_uniform_matrix (CoglPipeline *pipeline,
  * @pipeline: A #CoglPipeline
  * @snippet: The #CoglSnippet to add to the vertex processing hook
  *
- * Adds a shader snippet that to @pipeline. The snippet will wrap
- * around or replace some part of the pipeline as defined by the hook
- * point in @snippet. Note that some hook points are specific to a
- * layer and must be added with cogl_pipeline_add_layer_snippet()
- * instead.
+ * Adds a shader snippet to @pipeline. The snippet will wrap around or
+ * replace some part of the pipeline as defined by the hook point in
+ * @snippet. Note that some hook points are specific to a layer and
+ * must be added with cogl_pipeline_add_layer_snippet() instead.
  *
  * Since: 1.10
  * Stability: Unstable