doc: Doxygen, you're shitty.

An unqualified link to Extensions.h will, guess what -- pick the last
Extension.h header that Doxygen came across. So in this case
Magnum/Vk/Extensions.h. With no warning or any other hint that something
MAYBE could be wrong. FFS.

src/Magnum/Audio/Context.h

@@ -359,8 +359,9 @@ class MAGNUM_AUDIO_EXPORT Context {
         /**
          * @brief Whether given extension is supported
          *
-         * Extensions usable with this function are listed in @ref Extensions
-         * namespace in header @ref Extensions.h. Example usage:
+         * Extensions usable with this function are listed in the
+         * @ref Extensions namespace in the @ref Magnum/Audio/Extensions.h
+         * header. Example usage:
          *
          * @snippet MagnumAudio.cpp Context-isExtensionSupported
          *

src/Magnum/GL/Context.h

@@ -634,8 +634,9 @@ class MAGNUM_GL_EXPORT Context {
          * @brief Whether given extension is supported
          *
          * Extensions usable with this function are listed in the
-         * @ref Extensions namespace in the @ref Extensions.h header and in the
-         * @ref opengl-support "OpenGL support tables". Example usage:
+         * @ref Extensions namespace in the @ref Magnum/GL/Extensions.h header
+         * and in the @ref opengl-support "OpenGL support tables". Example
+         * usage:
          *
          * @snippet MagnumGL.cpp Context-isExtensionSupported
          *