Review whether there needs to be some syncing up between §11.2.2 and §9.2.9 *Local variables*, §9.4 *Definite assignment*

This [comment in #873](https://github.com/dotnet/csharpstandard/pull/873#discussion_r2366396417), which follows the one quoted below, is part of this issue as well.

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I think there probably needs to be some syncing up between here and §9.2.9 *Local variables*, §9.4 *Definite assignment*, including the lifetime & scope of the variable – hardly anything is stated about these here:

- Line 90 below states the variable is assigned on a match
- **Informatively** line 121 states for the example that after the `if` the variable is in scope but not definitely assigned, there is no `else` in the example so the definite assignment status there is not covered.

_Originally posted by @Nigel-Ecma in https://github.com/dotnet/csharpstandard/pull/873#discussion_r2366395973_