Link to existing rules in compound_stmts.rst (GH-149811)

In gh-138418, `!` was added to links to rules that don't exist in
the docs, in order to silence broken link warnings.
However, productionlist doesn't parse the `!`, which ends up in
the rendered documentation. (It's possible that gh-127835 broke
the `!` support.)

Replace the names with ones that appear in docs:

- `star_named_expression` in the grammar corresponds to
  `flexible_expression` in the docs
- `star_named_expressions` in the grammar corresponds to
  `flexible_expression_list` in the docs
- `named_expression` in the grammar corresponds to
  `assignment_expression` in the docs

Having two sets of names isn't great of course. Consolidating them
is tracked in (subissues of) gh-127833.