replace <tag> with <name>

[mccanne]

Noah made this suggestion:

We might want to follow Git's lead here and use "name" in place of "tag" for these terms, for two reasons. First, a Git user's notion of tags is quite different from this one, and that might cause confusion. And second, we might want to add Git-style tags some day.

The command-line help descriptions and docs/lake/design.md need to be updated...

[philrz]

Semi-relevant: There was a draft effort in #3238 to support GIt-style tags.