Clarify docs on default GL header inclusion

The documentation claims that any extension loader library header
included before the GLFW one will prevent the inclusion of the
default OpenGL header.  In reality this only worked if the loader
defined the canonical desktop OpenGL __gl_h_ inclusion guard and even
then relied on the OpenGL header to detect this.

This is the companion to 105cf32e0b, which
added the preprocessor logic to check for many known OpenGL and OpenGL
ES inclusion guards and not even attempt to include the default header
if any are present.

Some clarification of the language around header inclusion has also been
attempted, including making GLFW_INCLUDE_NONE more prominent.

Fixes #1695.

README.md

@@ -133,6 +133,8 @@ information on what to include when reporting a bug.
  - Bugfix: Video modes with a duplicate screen area were discarded (#1555,#1556)
  - Bugfix: Compiling with -Wextra-semi caused warnings (#1440)
  - Bugfix: Built-in mappings failed because some OEMs re-used VID/PID (#1583)
+ - Bugfix: Some extension loader headers did not prevent default OpenGL header
+   inclusion (#1695)
  - [Win32] Added the `GLFW_WIN32_KEYBOARD_MENU` window hint for enabling access
            to the window menu
  - [Win32] Added a version info resource to the GLFW DLL