Merge pull request swiftlang#16589 from kitasuke/update_documentation_of_alloc_stack_for_sil

[Gardening] Update alloc_stack documentation in SIL.rst

Author: CodaFi

docs/SIL.rst

@@ -1910,9 +1910,7 @@ Allocates uninitialized memory that is sufficiently aligned on the stack
 to contain a value of type ``T``. The result of the instruction is the address
 of the allocated memory.
 
-If a type is runtime-sized, the compiler must emit code to potentially
-dynamically allocate memory. So there is no guarantee that the allocated
-memory is really located on the stack.
+``alloc_stack`` always allocates memory on the stack even for runtime-sized type.
 
 ``alloc_stack`` marks the start of the lifetime of the value; the
 allocation must be balanced with a ``dealloc_stack`` instruction to