Documentation: only named keys generate synthetic key releases

The current wording states that all keys have synthetic key release
events generated after focus is lost, but keys that aren't named
don't have any state held, so no such events are generated for them.

The new wording clarifies that only named keys have the events generated
for them.

(cherry picked from commit dbe810e403)

CONTRIBUTORS.md

@@ -254,6 +254,7 @@ video tutorials.
  - Jonas Ådahl
  - Lasse Öörni
  - Leonard König
+ - Grzesiek11
  - All the unmentioned and anonymous contributors in the GLFW community, for bug
    reports, patches, feedback, testing and encouragement
 

include/GLFW/glfw3.h

@@ -4553,9 +4553,9 @@ GLFWAPI void glfwSetCursor(GLFWwindow* window, GLFWcursor* cursor);
  *  [character callback](@ref glfwSetCharCallback) instead.
  *
  *  When a window loses input focus, it will generate synthetic key release
- *  events for all pressed keys.  You can tell these events from user-generated
- *  events by the fact that the synthetic ones are generated after the focus
- *  loss event has been processed, i.e. after the
+ *  events for all pressed named keys.  You can tell these events from
+ *  user-generated events by the fact that the synthetic ones are generated
+ *  after the focus loss event has been processed, i.e. after the
  *  [window focus callback](@ref glfwSetWindowFocusCallback) has been called.
  *
  *  The scancode of a key is specific to that platform or sometimes even to that