Agree on simultaneous executable graph submission wording

[EwanC]

Decide on wording for submitting an executable graph while another instance of the same executable graph is still executing.

The current SYCL_EXT_ONEAPI_GRAPH wording is

Support for submitting a graph for execution, before a previous execution has been
completed is backend specific. The runtime may throw an error.

While SYCL_EXT_CODEPLAY_GRAPHS wording is

There are no gurantees that more than one instance of graph will execute concurrently. Submitting a graph for execution, before a previous execution has been complete may result in serialized execution depending on the SYCL backend an characteristics of the graph.

With the current specified behaviour it is hard for the user to know if the runtime will throw an error or not when they write application code attempting this.

[EwanC]

For revision 1 of the spec, we have decided to ban this behaviour intel#5626 (comment)