Refine thread safety language and track as part of XML

This issue follows on from discussion on [command-buffers and thread safety](https://github.com/KhronosGroup/OpenCL-Docs/issues/1323#issuecomment-2749777068) where it was decided that a better way to specify the thread-safety of OpenCL objects/entry-points was by:
* Moving the description of thread-safety from [the appendix](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#_multiple_host_threads) into the main spec
* Adopt a more precise term like "externally synchronized"
* Record this information in the XML file

Start by reading the discussion in issue 1323 and the current “Multiple Host Threads” appendix in the OpenCL specification. Identify where the thread-safety description should move in the main spec and how the XML records object or entry-point metadata. Done means the terminology is made precise, the main specification contains the guidance, and the XML tracks it consistently.

Written by the indexing model from the issue text.