なんかググってもあまりまとまった情報が出てこない（調べ方が悪いのかも。。）
普通にコマンドヘルプを見るのが良かった。
bundle exec rails --help
で一覧が表示できる
サブコマンドのヘルプを見たいときは以下のようにする。
bundle exec rails db:create --help
ちなみに一覧は以下（rails 6.0.0）
$ bundle exec rails --help
The most common rails commands are:
generate Generate new code (short-cut alias: "g")
console Start the Rails console (short-cut alias: "c")
server Start the Rails server (short-cut alias: "s")
test Run tests except system tests (short-cut alias: "t")
test:system Run system tests
dbconsole Start a console for the database specified in config/database.yml
(short-cut alias: "db")
new Create a new Rails application. "rails new my_app" creates a
new application called MyApp in "./my_app"
All commands can be run with -h (or --help) for more information.
In addition to those commands, there are:
about
action_mailbox:ingress:exim
action_mailbox:ingress:postfix
action_mailbox:ingress:qmail
action_mailbox:install
action_text:install
active_storage:install
app:template
app:update
assets:clean[keep]
assets:clobber
assets:environment
assets:precompile
cache_digests:dependencies
cache_digests:nested_dependencies
credentials:edit
credentials:show
db:create
db:drop
db:environment:set
db:fixtures:load
db:migrate
db:migrate:status
db:prepare
db:rollback
db:schema:cache:clear
db:schema:cache:dump
db:schema:dump
db:schema:load
db:seed
db:seed:replant
db:setup
db:structure:dump
db:structure:load
db:system:change
db:version
destroy
dev:cache
encrypted:edit
encrypted:show
initializers
log:clear
middleware
notes
restart
routes
runner
secret
secrets:edit
secrets:setup
secrets:show
spec
spec:domains
stats
test:db
time:zones[country_or_offset]
tmp:clear
tmp:create
version
webpacker
webpacker:binstubs
webpacker:check_binstubs
webpacker:check_node
webpacker:check_yarn
webpacker:clobber
webpacker:compile
webpacker:info
webpacker:install
webpacker:install:angular
webpacker:install:coffee
webpacker:install:elm
webpacker:install:erb
webpacker:install:react
webpacker:install:stimulus
webpacker:install:typescript
webpacker:install:vue
webpacker:verify_install
webpacker:yarn_install
yarn:install
zeitwerk:check
