users_guide: Fix various issues
users_guide: Fix various issues

Sphinx apparently expects references to anchors that don't accompany a
header to have a caption.

Test Plan: validate

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2033

ezyang/ghc,sgillespie/ghc,mettekou/ghc,olsner/ghc,sdiehl/ghc,sgillespie/ghc,vikraman/ghc,shlevy/ghc,vTurbine/ghc,vTurbine/ghc,olsner/ghc,ezyang/ghc,mettekou/ghc,sgillespie/ghc,vTurbine/ghc,vTurbine/ghc,vikraman/ghc,olsner/ghc,mettekou/ghc,shlevy/ghc,snoyberg/ghc,tjakway/ghcjvm,vTurbine/ghc,snoyberg/ghc,sdiehl/ghc,olsner/ghc,shlevy/ghc,vTurbine/ghc,mettekou/ghc,ezyang/ghc,olsner/ghc,sdiehl/ghc,tjakway/ghcjvm,sdiehl/ghc,snoyberg/ghc,tjakway/ghcjvm,vTurbine/ghc,olsner/ghc,snoyberg/ghc,mettekou/ghc,shlevy/ghc,ezyang/ghc,tjakway/ghcjvm,shlevy/ghc,sdiehl/ghc,olsner/ghc,vikraman/ghc,mettekou/ghc,sgillespie/ghc,vikraman/ghc,mettekou/ghc,tjakway/ghcjvm,tjakway/ghcjvm,sdiehl/ghc,shlevy/ghc,ezyang/ghc,vikraman/ghc,shlevy/ghc,sgillespie/ghc,tjakway/ghcjvm,snoyberg/ghc,sgillespie/ghc,vikraman/ghc,snoyberg/ghc,vikraman/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,snoyberg/ghc,sgillespie/ghc