fix(repo): correct default visibility in repo create help

Description

The help for glab repo create --internal says the flag is the default:

--internal   Make project internal: visible to any authenticated user. Default.

That is not what happens. runCreateProject starts with an empty visibility and only sets opts.Visibility when --internal, --private, or --public is passed. With no flag, glab sends no visibility field, and GitLab gives the new project the instance's default project visibility (Gitlab::CurrentSettings.default_project_visibility, see Project#initialize in app/models/project.rb). On GitLab.com, and on most instances, that is private, not internal.

So a user who reads the help and leaves out the flag, expecting an internal project, gets a different visibility.

This MR:

• Removes "Default." from the --internal flag help.
• Adds one sentence to the command description: if you don't pass --private, --internal, or --public, the project uses the default project visibility of your GitLab instance.
• Regenerates docs/source/repo/create.md with make gen-docs.

No behaviour change.

Related Issues

None found. I searched open and closed issues and merge requests for "repo create visibility", "project create visibility", "default visibility", "internal visibility", and "visibility".

How has this been tested?

• go run ./cmd/gen-docs/docs.go (same as make gen-docs): only docs/source/repo/create.md changed.
• go build ./...
• go vet ./internal/commands/project/create/
• go test ./internal/commands/project/create/ ./cmd/gen-docs/ pass.
• Read the code path in internal/commands/project/create/project_create.go: opts.Visibility is only set when visibility != "".

This contribution was prepared with the help of an AI assistant. I reviewed the changes and take responsibility for them.