jit: document function pointers

jit: document function pointers

gcc/jit/ChangeLog:
	* docs/topics/expressions.rst (Function calls): Add link to
	gcc_jit_context_new_function_ptr_type.
	(Function pointers): Convert to cross-references to
	function-pointers.rst, moving material there.
	* docs/topics/function-pointers.rst: New page.
	* docs/topics/index.rst: Add function-pointers.rst.
	* docs/topics/types.rst (Function pointer types): New section.
	* docs/_build/texinfo/libgccjit.texi: Regenerate.

From-SVN: r253257