Make Function member comments align with Class/Interface (#3727)

In `Class` we have:

```
  // The following members always have values, and do not change throughout the
  // lifetime of the class.

  // The following members are set at the `{` of the class definition.

  // The following members are accumulated throughout the class definition.

  // The following members are set at the `}` of the class definition.
```

`Interface` has similar, minus the "accumulated" members. I'm echoing
this, except `Function` has no `}` members; at present, nothing
differentiates between "started definition" and "completed definition",
unlike the other two.

Also removing a slightly inconsistent default value for decl_id.

toolchain/check/context.cpp

@@ -513,6 +513,7 @@ auto Context::FinalizeGlobalInit() -> void {
     sem_ir().functions().Add(
         {.name_id = SemIR::NameId::ForIdentifier(name_id),
          .enclosing_scope_id = SemIR::NameScopeId::Package,
+         .decl_id = SemIR::InstId::Invalid,
          .implicit_param_refs_id = SemIR::InstBlockId::Empty,
          .param_refs_id = SemIR::InstBlockId::Empty,
          .return_type_id = SemIR::TypeId::Invalid,