Disambiguate Vulkan support reference link

(cherry picked from commit 41a19ed49c5ace44949f58b157714d59c2aebf2f)

docs/vulkan.dox

@@ -14,7 +14,7 @@ To develop for Vulkan you should download the [LunarG Vulkan
 SDK](https://vulkan.lunarg.com/) for your platform.  Apart from headers and link
 libraries, they also provide the validation layers necessary for development.
 
-For details on a specific function in this category, see the @ref vulkan.  There
+For details on a specific Vulkan support function, see the @ref vulkan.  There
 are also guides for the other areas of the GLFW API.
 
  - @ref intro_guide

include/GLFW/glfw3.h

@@ -52,7 +52,7 @@ extern "C" {
  *  This is the reference documentation for OpenGL and OpenGL ES context related
  *  functions.  For more task-oriented information, see the @ref context_guide.
  */
-/*! @defgroup vulkan Vulkan reference
+/*! @defgroup vulkan Vulkan support reference
  *  @brief Functions and types related to Vulkan.
  *
  *  This is the reference documentation for Vulkan related functions and types.