Tidy up BuildTarget docs, messages and internal naming
Tidy up BuildTarget docs, messages and internal naming

Demote TargetString document TargetSelector. Reorder TargetString
section. Rename use of userTarget naming convention to match new type
names.

Drop use of term "build" from error messages since these errors will
appear for many commands, not just the build command.

mydaum/cabal,themoritz/cabal,mydaum/cabal,themoritz/cabal,themoritz/cabal