Imrpove help text and documentation of `pub get --enforce-lockfile`

[Sigurd Meldgaard]

currently it says:

> dart pub get --help
Get the current package's dependencies.

Usage: dart pub get
[...snip...]
    --enforce-lockfile    Enforce pubspec.lock. Fail resolution if pubspec.lock does not satisfy pubspec.yaml
[...snip...]

Could be more precise..

https://dart.dev/tools/pub/cmd/pub-get#--enforce-lockfile says:

Refuses to resolve dependencies with an error message if the pubspec.lock file deviates or is missing.

This is also vague and says nothing about the intended use.