Add "at risk" notice to SHACL-SPARQL prebinding

[afs]

https://www.w3.org/TR/shacl12-sparql/#pre-binding

Add an "feature at risk" notice to SHALCL SPARQL prebinding.
Say the feature may change (to clarify it is not going to be removed).

This gives the working group more space to realign it with work happening in the RDF & SPARQL working group in this area.