gh-135617: Clarify starred targets in with statements - #159838

Clarify the constraint on starred targets after with ... as: a starred target must be part of a list or tuple target. Show the valid [*items] and (*items,) forms and explain that a bare *items is invalid.

Link the rule to assignment targets without adding a separate grammar production.

Fixes #135617.

Validation:

• Compiled and executed five valid target forms and confirmed that three invalid starred forms raise SyntaxError on the current CPython build.
• Full HTML documentation build with warnings treated as errors.
• Sphinx lint and patchcheck.

• Issue: with statement star target #135617