[MLIR][SPIRVToLLVM] Updated the documentation for type conversion

[georgemitenkov]

Summary

This patch updates the type conversion section of the documentation.
It includes the modelling of array strides and the mapping of the
naturally padded structs.

mlir/docs/SPIRVToLLVMDialectConversion.md

 
 ### Array types
 
 SPIR-V distinguishes between array type and run-time array type, the length of
 which is not known at compile time. In LLVM, it is possible to index beyond the
 end of the array. Therfore, runtime array can be implemented as a zero length
 array type.
 
-Moreover, SPIR-V supports the notion of array stride. Note that this is **not**
-supported by type conversion at the moment.
+Moreover, SPIR-V supports the notion of array stride. Currently only natural
+strides (based on [`VulkanLayoutUtils`](VulkanLayoutUtils)) are supported. They
+are also mapped to LLVM array.
 
 SPIR-V Dialect                        | LLVM Dialect
 :-----------------------------------: | :-----------------------------------:
 `!spv.array<<count> x <element-type>>`| `!llvm.array<<count> x <element-type>>`
 `!spv.rtarray< <element-type> >`      | `!llvm.array<0 x <element-type>>`
 
 ### Struct types
 
 Members of SPIR-V struct types may have decorations and offset information.
 Currently, there is **no** support of member decorations conversion for structs.
 For more information see section on [Decorations](#Decorations-conversion). 
 
 Usually we expect that each struct member has a natural size and alignment.
 However, there are cases (*e.g.* in graphics) where one would place struct 
 members explicitly at particular offsets. This case is **not** supported
 at the moment. Hence, we adhere to the following mapping:
 
 *   Structs with no offset are modelled as LLVM packed structures.
 
 *   Structs with natural offset (*i.e.* offset that equals to cumulative size of
     the previous struct elements or is a natural alignment) are mapped to
-    naturally padded structs. Nested structs with offset and structs with offset
-    containing arrays are **not** supported.
+    naturally padded structs.
 
 *   Structs with unnatural offset (*i.e.* offset that is not equal to cumulative
     size of the previous struct elements) are **not** supported. In this case,
     offsets can be emulated with padding fields (*e.g.* integers). However, such
     a design would require index recalculation in the conversion of ops that
     involve memory addressing.
 
 Examples of SPIR-V struct conversion are:

 `spv._module_end` is mapped to an equivalent terminator `ModuleTerminatorOp`.
 
 ## `mlir-spirv-cpu-runner`
 
 **Note: this is a section in progress, more information will appear soon**
 
 [LLVMFunctionAttributes]: https://llvm.org/docs/LangRef.html#function-attributes
 [SPIRVFunctionAttributes]: https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#_a_id_function_control_a_function_control
+[VulkanLayoutUtils]: https://github.com/llvm/llvm-project/blob/master/mlir/include/mlir/Dialect/SPIRV/LayoutUtils.h