nsc: improve command documentation

[hugosantos]

Summary

• make nsc command summaries concise and action-oriented
• standardize terminology, capitalization, grammar, and punctuation
• add missing summaries for hidden command groups and correct misleading descriptions

Testing

• go test ./cmd/nsc/... ./internal/cli/cmd/admin ./internal/cli/cmd/auth ./internal/cli/cmd/aws ./internal/cli/cmd/gcp ./internal/cli/cmd/cluster/... ./internal/cli/cmd/devbox ./internal/cli/cmd/integrations ./internal/cli/cmd/scratch ./internal/cli/cmd/sdk ./internal/cli/cmd/token ./internal/cli/cmd/vault ./internal/cli/cmd/version ./internal/cli/cmd/workspace
• go run ./cmd/nsc/gendoc /tmp/nscdocs
• git diff --check