spec source for provisional extensions (#695)

* spec source for provisional extensions:

cl_khr_semaphore
cl_khr_external_semaphore and related extensions
cl_khr_external_memory and related extensions

* Apply editorial suggestions from code review

Co-authored-by: Jim Lewis <j.lewis1@samsung.com>

* Apply suggestions from code review

Co-authored-by: Alastair Murray <alastair.murray@codeplay.com>

Co-authored-by: Jim Lewis <j.lewis1@samsung.com>
Co-authored-by: Alastair Murray <alastair.murray@codeplay.com>

Author: 3 people

OpenCL_Ext.txt

@@ -84,6 +84,11 @@ include::ext/cl_khr_suggested_local_work_size.asciidoc[]
 
 include::ext/cl_khr_integer_dot_product.asciidoc[]
 
+include::ext/cl_khr_semaphore.asciidoc[]
+
+include::ext/cl_khr_external_semaphore.asciidoc[]
+include::ext/cl_khr_external_memory.asciidoc[]
+
 // NOTE: To keep meaningful section numbers, new
 // extension documents should be added above here!