Spec clarification for valid pFd return values in vkGetSemaphoreFdKHR

In the spec of VkImportSemaphoreFdInfoKHR it is stated:

• If handleType is VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_SYNC_FD_BIT, the special value -1 for fd is treated like a valid sync file descriptor referring to an object that has already signaled
• It is consistent with the option for implementations to return a -1 file descriptor when exporting a VK_EXTERNAL_SEMAPHORE_HANDLE_TYPE_SYNC_FD_BIT from a VkSemaphore which is signaled

But vkGetSemaphoreFdKHR does not contain information about pFd's value -1 being valid option. Or really any other information about valid values beside being integer.

This has led to a bug in Chromium expecting pFd value to be always >= 0 when the result is VK_SUCCESS which has caused issues on Huawei devices whose driver returns -1 for already signaled semaphore.

Start with the VkImportSemaphoreFdInfoKHR and vkGetSemaphoreFdKHR reference pages linked in the issue, comparing their descriptions of sync-fd values and successful exports. Clarify the vkGetSemaphoreFdKHR documentation so the valid -1 result for an already-signaled semaphore is explicit, then verify the wording is consistent across both pages.