Note that SDL_getenv() is case sensitive

Fixes https://github.com/libsdl-org/SDL/issues/15086

include/SDL3/SDL_stdinc.h

@@ -1796,6 +1796,8 @@ extern SDL_DECLSPEC void SDLCALL SDL_DestroyEnvironment(SDL_Environment *env);
 /**
  * Get the value of a variable in the environment.
  *
+ * The name of the variable is case sensitive on all platforms.
+ *
  * This function uses SDL's cached copy of the environment and is thread-safe.
  *
  * \param name the name of the variable to get.