rustc_target: move comment to macro

Avoid duplicating this comment in preparation for adding more enums.

Author: tamird

compiler/rustc_target/src/lib.rs

@@ -137,6 +137,14 @@ macro_rules! target_spec_enum {
                 $( #[$variant_attr:meta] )*
                 $Variant,
             )*
+            /// The vast majority of the time, the compiler deals with a fixed
+            /// set of values, so it is convenient for them to be represented in
+            /// an enum. However, it is possible to have arbitrary values in a
+            /// target JSON file (which can be parsed when `--target` is
+            /// specified). This might occur, for example, for an out-of-tree
+            /// codegen backend that supports a value (e.g. architecture or OS)
+            /// that rustc currently doesn't know about. This variant exists as
+            /// an escape hatch for such cases.
             $( #[$other_variant_attr] )*
             $OtherVariant(crate::spec::StaticCow<str>),
         }

compiler/rustc_target/src/spec/mod.rs

@@ -1884,13 +1884,6 @@ crate::target_spec_enum! {
         X86_64 = "x86_64",
         Xtensa = "xtensa",
     }
-    /// The vast majority of the time, the compiler deals with a fixed set of
-    /// target architectures, so it is convenient for them to be represented in
-    /// an enum. However, it is possible to have arbitrary values for the "arch"
-    /// field in a target JSON file (which can be parsed when `--target` is
-    /// specified). This might occur, for example, for an out-of-tree codegen
-    /// backend that supports an architecture that rustc currently doesn't know
-    /// about. This variant exists as an escape hatch for such cases.
     other_variant = Unknown;
 }