fix immersed manifold cell orientation doc
attempt to improve FS/VFS scalar-valued vs vector-valued mess
FIAT -> FFC/FIAT
small mixed rewrite
integral spacing fixes
minor extrusion demo changes
fix splelling
Merge pull request #318 from firedrakeproject/doc-fs-fixes

* doc-fs-fixes:
  fix splelling
  minor extrusion demo changes
  integral spacing fixes
  small mixed rewrite
  FIAT -> FFC/FIAT
  attempt to improve FS/VFS scalar-valued vs vector-valued mess
  fix immersed manifold cell orientation doc