include: Removed :: from symbol names in comments.

This isn't C++ code, so there's no need to append global symbols with two
colons. It looks ugly.

I _did_ leave them for actual C++ things in WinRT-specific comments and other
places, like a reference to a Perl class thing.

Also, even though it's not valid C, it's a useful expression to say
`StructType::SpecificField`, so I left those alone, too.

include/SDL3/SDL_surface.h

@@ -45,7 +45,7 @@ extern "C" {
 /**
  *  \name Surface flags
  *
- *  These are the currently supported flags for the ::SDL_Surface.
+ *  These are the currently supported flags for an SDL_Surface.
  *
  *  \internal
  *  Used internally (read-only).