GL: hide deprecated enum alias value from Doxygen.

Due to \m_enum_values_as_keywords it caused the Half value of the
deprecated HalfFloat to appear first in search as an alias.

src/Magnum/GL/Attribute.h

@@ -219,7 +219,11 @@ template<UnsignedInt location, class T> class Attribute {
              * Half float.
              * @m_deprecated_since{2020,06} Use @ref DataType::Half instead.
              */
-            HalfFloat CORRADE_DEPRECATED_ENUM("use Half instead") = Half,
+            HalfFloat CORRADE_DEPRECATED_ENUM("use Half instead")
+                #ifndef DOXYGEN_GENERATING_OUTPUT
+                = Half /* it messes up with @m_enum_values_as_keywords */
+                #endif
+                ,
             #endif
             #endif
 
@@ -539,7 +543,11 @@ class MAGNUM_GL_EXPORT DynamicAttribute {
              * Half float.
              * @m_deprecated_since{2020,06} Use @ref DataType::Half instead.
              */
-            HalfFloat CORRADE_DEPRECATED_ENUM("use Half instead") = Half,
+            HalfFloat CORRADE_DEPRECATED_ENUM("use Half instead")
+                #ifndef DOXYGEN_GENERATING_OUTPUT
+                = Half /* it messes up with @m_enum_values_as_keywords */
+                #endif
+                ,
             #endif
             #endif
 

src/Magnum/GL/PixelFormat.h

@@ -449,7 +449,11 @@ enum class PixelType: GLenum {
      * Half float.
      * @m_deprecated_since{2020,06} Use @ref PixelType::Half instead.
      */
-    HalfFloat CORRADE_DEPRECATED_ENUM("use Half instead") = Half,
+    HalfFloat CORRADE_DEPRECATED_ENUM("use Half instead")
+        #ifndef DOXYGEN_GENERATING_OUTPUT
+        = Half /* it messes up with @m_enum_values_as_keywords */
+        #endif
+        ,
     #endif
 
     /**