xml scheme: Clarification for array pointer optionality (inheritance)

From https://registry.khronos.org/vulkan/specs/latest/registry.html#_attributes_of_member_tags

optional - optional.
Must be "true" or "false".
The default value if unspecified is "false".
"true" specifies that this member can contain a sentinel value
indicating that no corresponding object or explicit value is referred
to: NULL for pointers, VK_NULL_HANDLE for handles, or 0 for other
scalar types.
"false" specifies that the member must contain a value object
reference or explicit value.
If the member is a pointer, multiple values may be provided, separated
by commas - one for each pointer indirection.
Structure members with name pNext must always be specified with
optional="true", since there is no requirement that any member of a
pNext chain have a following member in the chain.

optional - optional. Must be "true" or "false". The default value if unspecified is "false". "true" specifies that this member can contain a sentinel value indicating that no corresponding object or explicit value is referred to: NULL for pointers, VK_NULL_HANDLE for handles, or 0 for other scalar types. "false" specifies that the member must contain a value object reference or explicit value. If the member is a pointer, multiple values may be provided, separated by commas - one for each pointer indirection. Structure members with name pNext must always be specified with optional="true", since there is no requirement that any member of a pNext chain have a following member in the chain.
Note

While the optional attribute can be used for scalar types such as integers, it does not affect the output generators included with the [Vulkan API Specification](https://registry.khronos.org/vulkan/specs/latest/registry.html#vulkan-spec). In this case, the attribute serves only as an indicator to human readers of the XML.

Explicitly specifying optional="false" is not supported, but optional="false,true" is supported for a pointer type.

Btw I see the mention of `..it does not affect the output generators included with the Vulkan API Specification. In this case, the attribute serves only as an indicator to human readers of the XML...` part. But it could actually provide some benefits for language bindings (and yeah I also know that lang bindings is very unprioritized but still)

---

The `vk.xml` extensively uses links between the array pointer member and int member storing its size.

Simplest example is `VkDeviceCreateInfo` and extension enabling:

```xml

...
uint32_t enabledExtensionCount
const char* const* ppEnabledExtensionNames
...

```

We have `enabledExtensionCount` which is optional and we have `ppEnabledExtensionNames` which is not optional but links to the `enabledExtensionCount` (which is again optional). But it is common knowledge you can pass null to `ppEnabledExtensionNames`. Well seems like `ppEnabledExtensionNames` is actually "implicitly" `optional=true,false` but the [spec](https://registry.khronos.org/vulkan/specs/latest/registry.html#_attributes_of_member_tags) mentions that if `optional` is omitted it is value is by default `false`. The `vulkan_object.py` tells me that `ppEnabledExtensionNames` is `optional=false` but it is clear (maybe I am wrong) that it actually should be `optional=true,false`. It all seems like a general rule but I don't know maybe there are exceptions.

But if this is really a general rule I think that either:
1. Spec description and `vulkan_object.py` should reflect this **OR**
2. `vk.xml` be explicit and say if it is optional for each array linking to it is length ... (And yeah again I already know that lang binds are not in priority so 1 option seems better)

P.S: and one more thing. Counter example where `pGeometries` is explicitly set to `optional`:

```xml

...
uint32_t geometryCount
const VkAccelerationStructureGeometryKHR* pGeometries
const VkAccelerationStructureGeometryKHR* const* ppGeometries
...

```