[SYCL][ESIMD] Update ESIMD docs to address recent user comments: (#3516)

- Remove restriction on constant specialization
- Add support for shift operators

Author: kychendev

sycl/doc/extensions/ExplicitSIMD/README.md

@@ -86,7 +86,6 @@ program behavior if violated.
 ##### Unsupported standard SYCL APIs:
 - Local accessors
 - Most of image APIs
-- Specialization constants
 - Memory access through a raw pointer returned by `sycl::accessor::get_pointer()`
 
 ##### Other restrictions:

sycl/doc/extensions/ExplicitSIMD/dpcpp-explicit-simd.md

@@ -113,8 +113,8 @@ standard subscript operator ```[]```, which returns by value.
 
 For simd type object, Explicit SIMD supports the following simd vector operations:
 - Unary operators: ++ (*pre-/post-increment*), -- (*pre-/post-decrement*)
-- Binary operators: +, -, *, /, &, |, ^
-- Compound assignments: +=, -=, *=, /=, &=, |=, ^=
+- Binary operators: +, -, *, /, &, |, ^, <<, >>
+- Compound assignments: +=, -=, *=, /=, &=, |=, ^=, <<=, >>=
 - Compare operators: >, >=, <, <=, ==, !=
 
 These are all element-wise operations, which apply a specified operation to the