doc: Use groups instead of curves in TLS documentation

doc: Use groups instead of curves in TLS documentation

With TLS 1.3 the concept of curves was renamed to groups.  Update
our wording to use groups instead of curves to make it clear what
the underlying GUC can support.

This was extracted from a slightly larger patch which also renamed
variables to match the new terminology.  Given that we are in beta
this portion was however left as a future excercise.

Author: Evan Si <evsi@amazon.com>
Reviewed-by: Ewan Young <kdbase.hack@gmail.com>
Discussion: https://postgr.es/m/23C40DD6-1C47-46FC-A746-8A1D8530AD3E@amazon.com
Backpatch-through: 18