Unification: documenting eta for pi-types and record types.
Unification: documenting eta for pi-types and record types.

We also align their type on (more) standard invariants.

coq/coq,silene/coq,SkySkimmer/coq,SkySkimmer/coq,coq/coq,Zimmi48/coq,ejgallego/coq,JasonGross/coq,coq/coq,ejgallego/coq,SkySkimmer/coq,ejgallego/coq,silene/coq,coq/coq,ejgallego/coq,JasonGross/coq,JasonGross/coq,SkySkimmer/coq,Zimmi48/coq,Zimmi48/coq,Zimmi48/coq,JasonGross/coq,silene/coq