Documentation work

Related to #1065.

docs/context.dox

@@ -103,6 +103,9 @@ Before you can make OpenGL or OpenGL ES calls, you need to have a current
 context of the correct type.  A context can only be current for a single thread
 at a time, and a thread can only have a single context current at a time.
 
+When moving a context between threads, you must make it non-current on the old
+thread before making it current on the new one.
+
 The context of a window is made current with @ref glfwMakeContextCurrent.
 
 @code

include/GLFW/glfw3.h

@@ -4809,10 +4809,13 @@ GLFWAPI uint64_t glfwGetTimerFrequency(void);
  *  thread.
  *
  *  This function makes the OpenGL or OpenGL ES context of the specified window
- *  current on the calling thread.  A context can only be made current on
+ *  current on the calling thread.  A context must only be made current on
  *  a single thread at a time and each thread can have only a single current
  *  context at a time.
  *
+ *  When moving a context between threads, you must make it non-current on the
+ *  old thread before making it current on the new one.
+ *
  *  By default, making a context non-current implicitly forces a pipeline flush.
  *  On machines that support `GL_KHR_context_flush_control`, you can control
  *  whether a context performs this flush by setting the