docs: clarify that named Arguments are excluded from cmd.Args()

[dearchap]

What this does

Closes #2387.

Issue #2387 reported that positional arguments consumed by named Arguments (e.g. &cli.StringArg{Name: "test"}) don't show up in cmd.Args(). This is intended behavior that dates back to the original arguments feature (#1074): each named argument consumes the positional arguments it needs, and cmd.Args() returns only the unconsumed (leftover) arguments. The consumed values are retrievable via cmd.{Type}Arg(name) (e.g. cmd.StringArg("test")).

This PR documents that behavior so it isn't a surprise:

• Clarify the Command.Args() doc comment (and regenerate godoc-current.txt / testdata/godoc-v3.x.txt).
• Add a "Mixing named arguments with cmd.Args()" section to docs/v3/examples/arguments/advanced.md with a runnable example and the trailing StringArgs{Max: -1} glob pattern.
• Add a note to docs/v3/examples/arguments/basics.md pointing to the advanced docs.
• Add TestArgsConsumedByNamedArguments to lock in the documented semantics.

Verification

• go test ./...
• go vet ./...
• make lint (goimports)
• make generate && make v3approve && make v3diff
• make gfmrun FLAGS='--walk docs/v3/' (error_count=0)
• mkdocs build succeeds