message: docs: better titles
diff: ## Registration
  -### Account activation
  +### Activate accounts
  By default, accounts are automatically activated on registration. You may want to change this so you add a step to the registration process.
  @@ -23,7 +23,7 @@ You can limit registration to ranges of emails that are only part of a whitelist
  ALLOWED_EMAIL_DOMAINS=gmail.com,yourorganisation.com
  ```
  -### Minimum password length
  +### Password constraints
  By default, clients can register with a password of at least three characters. You can change this in setting a higher value:
  @@ -31,15 +31,13 @@ By default, clients can register with a password of at least three characters. Y
  MIN_PASSWORD_LENGTH=6
  ```
  -### Check password simplicity against Have I Been Pwned
  -
  You can ask HBP to check on [Have I Been Pwned](https://haveibeenpwned.com/Passwords) if the password has been previously exposed in data breaches. If so, the registration will fail. This option is disabled by default.
  ```
  HIBP_ENABLE=false
  ```
  -### Add registration fields
  +### Additional registration fields
  You may want to extend the `public.users` table with your own fields and relations, and to expect the client to set some of them when registering. It is possible to set a list of columns in the `REGISTRATION_CUSTOM_FIELDS` environment value.
  