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.
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@253257 138bc75d-0d04-0410-961f-82ee72b054a4