The previous documentation was inaccurate, stating that it would not update existing inputs. However these inputs will be updated if they are outdated (for example the version of an existing input has been changed). The new text properly reflects this behaviour.
This is probably what most people expect it to do. Fixes#3781.
There is a new command 'nix flake lock' that has the old behaviour of
'nix flake update', i.e. it just adds missing lock file entries unless
overriden using --update-input.
