singles: update MagnumMath docs with what CorradeCpu has already.

In particular:

 - listing the *very important* stuff -- such as what macro to define to
   make the library usable at all -- in the header
 - mentioning what to do if cross-SO/DLL visibility is desired

The actual generated single-header file will be updated once I find more
time. Not now.

doc/namespaces.dox

@@ -109,8 +109,10 @@ more information.
     #include <MagnumMath.hpp>
     @endcode
 @par
-    In addition, contents of the @ref GlmIntegration and @ref EigenIntegration
+    If you need the deinlined symbols to be exported from a shared library,
-    libraries are included as well --- opt-in by specifying either
+    @cpp #define MAGNUM_EXPORT @ce as appropriate. In addition, contents of the
+    @ref GlmIntegration and @ref EigenIntegration libraries are included as
+    well --- opt-in by specifying either
     @cpp #define MAGNUM_MATH_GLM_INTEGRATION @ce or
     @cpp #define MAGNUM_MATH_EIGEN_INTEGRATION @ce before including the file.
     Including it multiple times with different macros defined works as well.

src/singles/MagnumMath.hpp

@@ -15,6 +15,19 @@
     -   GitHub project page — https://github.com/mosra/magnum
     -   GitHub Singles repository — https://github.com/mosra/magnum-singles
 
+    The library has a separate non-inline implementation part, enable it *just
+    once* like this:
+
+        #define MAGNUM_MATH_IMPLEMENTATION
+        #include <MagnumMath.hpp>
+
+    If you need the deinlined symbols to be exported from a shared library,
+    `#define MAGNUM_EXPORT` as appropriate. In addition, contents of the
+    GlmIntegration and EigenIntegration libraries are included as well ---
+    opt-in by specifying either `#define MAGNUM_MATH_GLM_INTEGRATION` or
+    `#define MAGNUM_MATH_EIGEN_INTEGRATION` before including the file.
+    Including it multiple times with different macros defined works as well.
+
     v2020.06-0-gfac6f4da2 (2020-06-27)
     -   Various fixes for Clang-CL compatibility
     -   Expanding the APIs to work with Half and long double types