Add note that get_devices(host) returns nothing

Although device_type::host still exists, the concept of a "host device" does
not exist. Calling get_devices() with device_type::host should not do anything.

Author: Pennycook

adoc/chapters/programming_interface.adoc

@@ -1129,6 +1129,10 @@ _Returns:_ A [code]#std::vector# containing all of the <<root-device,root
 devices>> associated with this platform which have the device type specified by
 [code]#type#.
 
+{note}Since the concept of a "host device" does not exist in SYCL 2020, if
+[code]#type# is [code]#info::device_type::host# this function will always return
+an empty vector.{endnote}
+
 _Remarks_: If [code]#type# is [code]#info::device_type::all#, the
 [code]#std::vector# contains all root devices in this platform.
 If [code]#type# is [code]#info::device_type::automatic# and the platform is not
@@ -1851,6 +1855,10 @@ _Returns:_ A [code]#std::vector# containing all the <<root-device, root
 devices>> from all <<backend, SYCL backends>> available in the system which have
 the device type [code]#type#.
 
+{note}Since the concept of a "host device" does not exist in SYCL 2020, if
+[code]#type# is [code]#info::device_type::host# this function will always return
+an empty vector.{endnote}
+
 _Remarks_: If [code]#type# is [code]#info::device_type::all#, the
 [code]#std::vector# contains all root devices in the system.
 If [code]#type# is [code]#info::device_type::automatic#, the [code]#std::vector#