docs: clarify token creation in npm login command

[LiangYingC]

This is simple PR for adjust npm login doc.

Problem Description:

When using npm login, users are not aware that this command generates a legacy token with publish permissions. This information is not evident in the npm login cli documentation.

After a while searching. found that this context and detail hiding in the About legacy tokens documentation.

Solution:

Add a clear description in the npm login documentation in this PR so that users can understand publish token would be created after npm login in the future.

References

[wraithgar]

This is a great addition, however that file is pulled externally from the cli repo and needs to be changed there, not here. We may also want to add a similar clause to adduser

[LiangYingC]

@wraithgar thanks for your reminders. I've opened another PR to make this addition.

npm/cli#8226