[SYCL][DOC] Add validation rules to SPIR-V ext (#5687)

Add validation rules to the SPIR-V extension
SPV_INTEL_global_variable_decorations.

Author: gmlueck

sycl/doc/design/spirv-extensions/SPV_INTEL_global_variable_decorations.asciidoc

@@ -207,7 +207,8 @@ Modify Section 3.31, Capability, adding a row to the Capability table:
 
 === Validation Rules
 
-None.
+* It is invalid for two *HostAccessINTEL* decorations in the same module to
+  have the same _Name_ operand.
 
 == Issues