howto.html: Update commentary.

howto.html: Update commentary.

	* docs/html/ext/howto.html: Update commentary.
	* include/bits/c++config: Update threading configuration comment.
	(__STL_GTHREADS): Remove macro definition.
	(__STL_THREADS): Likewise.
	* include/bits/stl_threads.h: Leave only the configuration
	path which had been guarded by __STL_GTHREADS.  Remove all
	guards related to __STL_GTHREADS, __STL_SGI_THREADS,
	__STL_PTHREADS, __STL_UITHREADS and __STL_WIN32THREADS.
	* include/bits/stl_alloc.h: Leave only the configuration path
	which had been guarded by __STL_THREADS.  Remove configuration
	path and guards for __STL_SGI_THREADS.
	(__NODE_ALLOCATOR_THREADS): Remove macro definition.  Unroll its use.
	(__NODE_ALLOCATOR_LOCK): Likewise.
	(__NODE_ALLOCATOR_UNLOCK): Likewise.
	(_NOTHREADS): Remove guards related to macro.
	* include/ext/stl_rope.h: Remove configuration path and guards
	for __STL_SGI_THREADS.
	* src/stl-inst.cc: Remove use of __NODE_ALLOCATOR_THREADS.

From-SVN: r47557