The documentation comment for LogMessageParserInterface::parseMessagePlaceholders() is the following one.
/**
* Parses and transforms message and its placeholders to a common format.
*
* For a value to be considered as a placeholder should be in the following
* formats:
* - PSR3 format:
* @see https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-3-logger-interface.md#12-message
* - Drupal specific string placeholder format:
* @see \Drupal\Component\Render\FormattableMarkup
*
* Values in PSR3 format will be transformed to
* \Drupal\Component\Render\FormattableMarkup format.
*
* @param string $message
* The message that contains the placeholders.
* If the message is in PSR3 style, it will be transformed to
* \Drupal\Component\Render\FormattableMarkup style.
* @param array $context
* An array that may or may not contain placeholder variables.
*
* @return array
* An array of the extracted message placeholders.
*/
The list about the placeholder formats is rendered as follows.
For a value to be considered as a placeholder should be in the following
formats:
- PSR3 format:
- Drupal specific string placeholder format:
The list items are shown indented, when they should not, and there isn't any link.
I am linking the Drupal 8.6 documentation because there isn't a link to the Drupal 8.7 documentation, so far, but the bug exists also for Drupal 8.7.