Feat:(issue_2392) Single-value arguments can be marked as required

[idelchi]

What type of PR is this?

• feature

What this PR does / why we need it:

• args.go: adds a Required field to ArgumentBase, same as flags have. A missing required argument returns required argument "name" not set. Usage() now renders optional single args as [name], matching what {Type}Args already does.
• args_test.go: adds TestSingleRequiredArg and TestChainedRequiredArgs. The TestArgUsage expectation for an optional arg changes from ia to [ia].
• docs/v3/examples/arguments/advanced.md: short section on required arguments.
• godoc-current.txt / testdata/godoc-v3.x.txt: regenerated.

A default Value does not satisfy Required, same as with flags.

Which issue(s) this PR fixes:

Fixes #2392

Special notes for your reviewer:

The help output change (optional single args rendering as [name]) is a separate commit and can be dropped if unwanted.

Testing

• go test ./...
• make gfmrun for the docs example
• make generate + make v3approve for the godoc diff check

Release Notes

Single-value arguments can be marked as `Required`. Optional single-value arguments now render as `[name]` in help output.