[SYCL][DOC] Update the 'constraints' for the complex specification (#12649)

This PR apply the modifications asked in the
#11792 PR made by @Pennycook

---------

Co-authored-by: John Pennycook <[email protected]>

Author: jle-quel

sycl/doc/extensions/experimental/sycl_ext_oneapi_complex.asciidoc

@@ -10,6 +10,7 @@
 :encoding: utf-8
 :lang: en
 :dpcpp: pass:[DPC++]
+:endnote: —{nbsp}end{nbsp}note
 
 // Set the default source code type in this document to C++,
 // for syntax highlighting purposes.  This is needed because
@@ -101,8 +102,8 @@ available only in host code as noted below.
 The complex type is trivially copyable and type trait `is_device_copyable`
 should resolve to `std::true_type`.
 
-The `T` template parameter must be one of the types float, double, or
-sycl::half.
+_Constraints_: The `T` template parameter must be one of the types `float`,
+`double`, or `sycl::half`.
 
 Note: When performing operations between complex numbers and decimals,
 the decimal is treated as a complex number with a real component equal to
@@ -335,6 +336,13 @@ Additionally, this extension introduces support for the `real` and `imag` free
 functions, which returns the real and imaginary component of a number,
 respectively.
 
+[_Note:_ The overloads of the functions `real(T)` and `imag(T)` match the
+behavior in ISO C++ where `T` would be treated as a complex number with a zero
+imaginary component. This is subject to the constraint that `T` must be one of
+the types `float`, `double`, `sycl::half`, or evaluate to `true` for
+`std::is_integral`.
+_{endnote}_]
+
 These functions are available in both host and device code, and each math
 function should follow the C++ standard for handling `NaN` and `Inf` values.