message: Fix typos and reword slightly
diff: @@ -49,7 +49,7 @@ id. Source is the place where the source code for package comes
  from. Typical sources are crates.io, a git repository or a folder on
  the local hard drive.
  -`Resolve` is the representation of a direct acyclic graph of package
  +`Resolve` is the representation of a directed acyclic graph of package
  dependencies, which uses `PackageId`s for nodes. This is the data
  structure that is saved to the lock file. If there is no lockfile,
  Cargo constructs a resolve by finding a graph of packages which
  @@ -69,11 +69,13 @@ the registry storage format.
  ## Concurrency
  -Cargo itself is mostly single hreaded, but it schedules compiler jobs
  -in parallel if possible. However there can be several different
  -instances of Cargo running concurrently on the system, so Cargo uses
  -file locking when accessing potentially shared data like the registry
  -or the target directory.
  +Cargo is mostly single threaded. The only concurrency inside a single
  +instance of Cargo happens during compilation, when several instances
  +of `rustc` are invoked in parallel to build independent
  +targets. However there can be several different instances of Cargo
  +process running concurrently on the system. Cargo guarantees that this
  +is always safe by using file locks when accessing potentially shared
  +data like the registry or the target directory.
  ## Tests
  