Replace "must be" with "is" in constraints

"is" is more consistent with ISO C++ wording.

adoc/extensions/sycl_khr_free_function_commands.adoc

@@ -533,7 +533,7 @@ void copy(sycl::queue q, const T* src, T* dest, size_t count);    (2)
 
 Copies between two USM pointers.
 
-_Constraints_: [code]#T# must be <<device-copyable>>.
+_Constraints_: [code]#T# is <<device-copyable>>.
 
 _Preconditions_: [code]#src# and [code]#dest# must be host pointers or USM
 pointers accessible on the device.
@@ -577,8 +577,8 @@ void copy(sycl::queue q,
 
 Copies from host to device.
 
-_Constraints_: [code]#SrcT# and [code]#DestT# must be <<device-copyable>>.
-[code]#DestMode# must be [code]#access_mode::write# or
+_Constraints_: [code]#SrcT# and [code]#DestT# is <<device-copyable>>.
+[code]#DestMode# is [code]#access_mode::write# or
 [code]#access_mode::read_write#.
 
 _Preconditions_: [code]#src# must be a host pointer, pointing to an allocation
@@ -621,8 +621,8 @@ void copy(sycl::queue q,
 
 Copies from device to host.
 
-_Constraints_: [code]#SrcT# and [code]#DestT# must be <<device-copyable>>.
-[code]#DestMode# must be [code]#access_mode::read# or
+_Constraints_: [code]#SrcT# and [code]#DestT# is <<device-copyable>>.
+[code]#DestMode# is [code]#access_mode::read# or
 [code]#access_mode::read_write#.
 
 _Preconditions_: [code]#dest# must be a host pointer, pointing to an allocation
@@ -657,10 +657,9 @@ void copy(sycl::queue q,
 
 Copies between two device accessors.
 
-_Constraints_: [code]#SrcT# and [code]#DestT# must be <<device-copyable>>.
-[code]#SrcMode# must be [code]#access_mode::read# or
-[code]#access_mode::read_write#.
-[code]#DestMode# must be [code]#access_mode::write# or
+_Constraints_: [code]#SrcT# and [code]#DestT# is <<device-copyable>>.
+[code]#SrcMode# is [code]#access_mode::read# or [code]#access_mode::read_write#.
+[code]#DestMode# is [code]#access_mode::write# or
 [code]#access_mode::read_write#.
 
 _Effects (11)_: Equivalent to [code]#h.copy(src, dest)#.
@@ -739,7 +738,7 @@ void update_host(sycl::queue q, accessor<T, Dims, Mode, target::device> acc);
 
 }
 ----
-_Constraints_: [code]#T# must be <<device-copyable>>.
+_Constraints_: [code]#T# is <<device-copyable>>.
 
 _Effects (1)_: Equivalent to [code]#h.update_host(acc)#.