message: Clarify FAQ entry wording about lockfiles
  I received a report that this FAQ entry (and in particular the wording
  "across whatever machine") generated some confusion. Reword the FAQ
  entry for clarity.
diff: @@ -103,10 +103,11 @@ issue][3].
  ### Why do binaries have `Cargo.lock` in version control, but not libraries?
  -The purpose of a `Cargo.lock` is to describe the state of the world at the time
  -of a successful build. It is then used to provide deterministic builds across
  -whatever machine is building the package by ensuring that the exact same
  -dependencies are being compiled.
  +The purpose of a `Cargo.lock` lockfile is to describe the state of the world at
  +the time of a successful build. Cargo uses the lockfile to provide
  +deterministic builds on different times and different systems, by ensuring that
  +the exact same dependencies and versions are used as when the `Cargo.lock` file
  +was originally generated.
  This property is most desirable from applications and packages which are at the
  very end of the dependency chain (binaries). As a result, it is recommended that
  