gccrs: expand: Change names and document behavior

gccrs: expand: Change names and document behavior

Change some argument name, as well as some documentation.

gcc/rust/ChangeLog:

	* ast/rust-ast.cc (BlockExpr::normalize_tail_expr): Refactor
	code and warn about dangling reference.
	* expand/rust-expand-visitor.cc (expand_stmt_attribute):
	Document function and change argument names to make those
	clearer.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>