doc: ignore CORRADE_NODISCARD annotations when generating docs.

I don't think my ancient Doxygen fork is aware of [[nodiscard]], and
it's quite clear that the return value is important when looking at the
docs, so just remove it from processing.

doc/Doxyfile

@@ -2262,7 +2262,7 @@ PREDEFINED             = DOXYGEN_GENERATING_OUTPUT \
     CORRADE_DEPRECATED_FILE(message)= CORRADE_VISIBILITY_EXPORT= \
     CORRADE_VISIBILITY_LOCAL= CORRADE_VISIBILITY_IMPORT= \
     CORRADE_IGNORE_DEPRECATED_PUSH= CORRADE_IGNORE_DEPRECATED_POP= \
-    CORRADE_ENUMSET_OPERATORS(message)= CORRADE_UNUSED= \
+    CORRADE_ENUMSET_OPERATORS(message)= CORRADE_UNUSED= CORRADE_NODISCARD= \
     MAGNUM_BUILD_DEPRECATED MAGNUM_TARGET_GL MAGNUM_TARGET_VK \
     MAGNUM_GL_ABSTRACTSHADERPROGRAM_SUBCLASS_DRAW_IMPLEMENTATION(type)= \
     MAGNUM_GL_ABSTRACTSHADERPROGRAM_SUBCLASS_DISPATCH_IMPLEMENTATION(type)= \