The D8 documentation standards at https://www.drupal.org/node/1354 state that for the built-in PHP data type NULL the string 'null' should be used in docblocks rather than 'NULL' when specifying a parameter or return value. Attached is a patch correcting a number of invalid formats that currently exist within core. Advice would be appreciated for how large patches like this can become before it is considered "disruptive".
This patch also includes a few docblock corrections for lines that were touched by the 'NULL' type changes. These include the correction of indentation and/or addition of '(optional)' description where needed.