Add a vision for improving the approachability of data-race safety. (#2621)

* Add a prospective vision for improving the approachability of
data-race safety.

* Specify approachability of data-race safety for programmers with other
concurrent programming experience as an explicit goal of the vision
document.

* Clarify why diagnostics and documentation are not included in the
vision document.

* Minor clarifications and typo fixes.

* Clarify how diagnostics are considered in the evolution process.

* Add boilerplate re acceptance

---------

Co-authored-by: Xiaodi Wu <[email protected]>

Author: hborla