Add description to rake generate:stdlib_test[class]

[ybiquitous]

The document stdlib.md describes the command rake generate:stdlib_test[class].
(see https://github.com/ruby/rbs/blob/018f671ed048975acf68fd40412f91e90600aa16/docs/stdlib.md#writing-tests)

So, I think it useful that the rake task is output via rake --tasks:

$ bundle exec rake --tasks
rake build                        # Build rbs-1.0.0.gem into the pkg directory
rake clean                        # Remove any temporary products
rake clobber                      # Remove any generated files
rake generate:stdlib_test[class]  # Generate a test file for a stdlib class signatures
rake install                      # Build and install rbs-1.0.0.gem into system gems
rake install:local                # Build and install rbs-1.0.0.gem into system gems without network access
rake release[remote]              # Create tag v1.0.0 and build and push rbs-1.0.0.gem to rubygems.org
rake test                         # Run tests

[pocke]

LGTM 🌟
Thanks!