Documentation tweaks

gcc/jit/ChangeLog.jit:
	* TODO.rst: Update.
	* docs/topics/expressions.rst (gcc_jit_context_new_call): Add
	a note clarifying the behavior of this entrypoint.
	* docs/topics/functions.rst (Creating and using functions): Markup
	fix.
	(gcc_jit_block_add_assignment_op): Add an example.