Mention availability of Application libraries on different platforms.

doc/building.dox

@@ -167,16 +167,23 @@ platform best:
 - `WITH_GLUTAPPLICATION` - @ref Platform::GlutApplication "GlutApplication",
   available only if targeting desktop OpenGL. Requires **GLUT** library.
 - `WITH_GLXAPPLICATION` - @ref Platform::GlxApplication "GlxApplication".
-  Requires **X11** and **GLX** libraries.
+  Available only if targetting desktop OpenGL or ES desktop emulation, not
+  available on Google Chrome Native Client or Emscripten. Requires **X11** and
+  **GLX** libraries.
 - `WITH_NACLAPPLICATION` - @ref Platform::NaClApplication "NaClApplication",
-  available only if targeting Google Chrome Native Client (see below).
+  available only if targeting Google Chrome Native Client.
 - `WITH_SDL2APPLICATION` - @ref Platform::Sdl2Application "Sdl2Application".
+  Available also on Emscripten, but not on Google Chrome Native Client.
   Requires **SDL2** library.
 - `WITH_XEGLAPPLICATION` - @ref Platform::XEglApplication "XEglApplication",
-  available only if targeting OpenGL ES (see above). Requires **X11** and **EGL**
-  libraries.
+  available only if targeting OpenGL ES (see above). Not available on Google
+  Chrome Native Client or Emscripten. Requires **X11** and **EGL** libraries.
 - `WITH_WINDOWLESSGLXAPPLICATION` - @ref Platform::WindowlessGlxApplication "WindowlessGlxApplication".
-  Requires **X11** and **GLX** libraries.
+  Available only if targetting desktop OpenGL or ES desktop emulation, not
+  available on Google Chrome Native Client or Emscripten. Requires **X11** and
+  **GLX** libraries.
+- `WITH_WINDOWLESSNACLAPPLICATION` - @ref Platform::WindowlessNaClApplication "WindowlessNaClApplication",
+  available only if targeting Google Chrome Native Client.
 
 @subsection building-tests Building and running unit tests