Minor improvements for Build_info and install

I noticed a few things about Build_info and related use of dune install that could be improved.

• The https://dune.readthedocs.io/en/latest/dune-libs.html#build-info section led me to add dune.build-info to the libraries entry in my executable stanza, but from reading the dune tests it turned out that dune-build-info is needed instead.

• The example code in that section doesn't compile since Build_info.V1.version now takes a unit arg.

• The Installing ... messages do not seem to respect e.g. --display=quiet.

• It would be nice to warn if setting --prefix=_build/install/... since doing so silently produces empty exes.

• dune install --help claims to support --watch but seems to ignore it.

Start with the Build_info section in the Dune documentation and the Dune tests referenced in the issue. Reproduce the remaining cases with --display=quiet, --prefix=_build/install/..., and --watch, then trace the install command handling. Done means the documented dependency and example are correct and each reported install behavior is addressed or clearly scoped.