Clarify MAGNUM_BUILD_DEPRECATED docs.

The glTF importer plugin contains functionality that's present only in
deprecated builds (backwards compatibility for skinning mesh
attributes), which isn't causing any API signature difference but rather
a difference in behavior. So expand the docs to say it's not limited to
just APIs but features in general.

doc/building.dox

@@ -677,11 +677,11 @@ Options controlling the build:
     versa. To some extent it's also possible to have only a subset of plugins
     built as static --- set `MAGNUM_<PLUGIN>_BUILD_STATIC` for particular
     plugins to `ON` or `OFF` to override this option.
--   `MAGNUM_BUILD_DEPRECATED` --- Include deprecated APIs in the build. Enabled
+-   `MAGNUM_BUILD_DEPRECATED` --- Build with deprecated features included.
-    by default to preserve backwards compatibility, disabling it forces you to
+    Enabled by default to preserve backwards compatibility, disabling it forces
-    update your code whenever there's a breaking API change. It's however
+    you to update your code whenever there's a breaking change in the APIs or
-    recommended to have this option disabled when deploying a final application
+    in library behavior. It's however recommended to have this option disabled
-    as it can result in smaller binaries.
+    when deploying a final application as it can result in smaller binaries.
 -   `MAGNUM_DISTANCEFIELDCONVERTER_STATIC_PLUGINS`,
     `MAGNUM_FONTCONVERTER_STATIC_PLUGINS`,
     `MAGNUM_IMAGECONVERTER_STATIC_PLUGINS`,

doc/cmake.dox

@@ -316,7 +316,7 @@ Features of found Magnum library are exposed in these CMake variables, they
 are also available as preprocessor variables if including
 @ref Magnum/Magnum.h "Magnum/Magnum.h":
 
--   `MAGNUM_BUILD_DEPRECATED` --- Defined if compiled with deprecated APIs
+-   `MAGNUM_BUILD_DEPRECATED` --- Defined if compiled with deprecated features
     included
 -   `MAGNUM_BUILD_STATIC` --- Defined if compiled as static libraries. Default
     are shared libraries.

modules/FindMagnum.cmake

@@ -128,7 +128,7 @@
 #
 # Features of found Magnum library are exposed in these variables:
 #
-#  MAGNUM_BUILD_DEPRECATED      - Defined if compiled with deprecated APIs
+#  MAGNUM_BUILD_DEPRECATED      - Defined if compiled with deprecated features
 #   included
 #  MAGNUM_BUILD_STATIC          - Defined if compiled as static libraries
 #  MAGNUM_BUILD_STATIC_UNIQUE_GLOBALS - Defined if static libraries keep the

src/Magnum/Magnum.h

@@ -87,11 +87,11 @@ using Corrade::Utility::Fatal;
 
 #ifdef DOXYGEN_GENERATING_OUTPUT
 /**
-@brief Build with deprecated API included
+@brief Build with deprecated features included
 
-Defined if the library contains deprecated API (which will be removed in the
+Defined if the library contains deprecated features and APIs (which will be
-future). To preserve backward compatibility, Magnum is by default built with
+removed in the future). To preserve backward compatibility, Magnum is by
-deprecated API included.
+default built with deprecated features included.
 @see @ref building, @ref cmake
 */
 /* This macro is enabled by default in Doxyfile, but we may still want to