remove the dummy typedef for struct SoupSessionFeature that was needed

remove the dummy typedef for struct SoupSessionFeature that was needed

	* libsoup/soup-session-feature.h: remove the dummy typedef for
	struct SoupSessionFeature that was needed because we weren't
	scanning soup-types.h. Fixes the build with non-GNU compilers.

	* libsoup/soup-session-feature.c: document
	SoupSessionFeatureInterface

	* docs/reference/Makefile.am (IGNORE_HFILES): oops, don't ignore
	soup-types.h

	* docs/reference/libsoup-2.4-docs.sgml:
	* docs/reference/libsoup-2.4-sections.txt: Split
	SoupSessionFeature into its own file.

svn path=/trunk/; revision=1231