Unified API Spec: Clean up 1.2 memory consistency description

During review of the first unified API spec PR there was a suggestion [1] that the 1.2 memory consistency description could be clearer than the original 1.2 specification wording without changing the meaning.

This seems sensible given that it now sits next to the extensive 2.0+ memory consistency descriptions.

Potential wording:

> Local memory is consistent across work-items in a single work-group at a work-group barrier if CLK_LOCAL_MEM_FENCE is set in the barrier memory fence flags.
> Global memory is consistent across work-items in a single work-group at a work-group barrier if CLK_GLOBAL_MEM_FENCE is set in the barrier memory fence flags.
> There are no guarantees of memory consistency between different work-groups executing a kernel.

[1] https://github.com/KhronosGroup/OpenCL-Docs/pull/78#discussion_r334103864