Clarify when max_size() was deprecated

Co-authored-by: Greg Lueck <gregory.m.lueck@intel.com>

Author: Pennycook

adoc/chapters/programming_interface.adoc

@@ -9453,7 +9453,7 @@ a@
 ----
 size_type max_size() const noexcept
 ----
-   a@ Deprecated.
+   a@ Deprecated by SYCL 2020.
 
 Returns the maximum number of elements any accessor of this type would be able
 to access.