Improve phpdoc types

[lekoala]

Description

I'd like to make the codebase easier to use with phpstan

Additional context or points of discussion

See here for more details: #11149

Validations

• You intend to implement the feature yourself
• You have read the contributing guide
• You strongly believe this feature should be in core, rather than being its own community module
• You have checked for existing issues or pull requests related to this feature (and didn't find any)

PRs

• ENH docblock types #11168
• ENH fieldList is an array or null #11177
• ENH Methods with missing nullable return types #11187
• ENH add nullable return types #11192

[GuySartorelli]

I've merged the linked PR.
@lekoala are there additional docblock types you want to update? Or should I close this issue as complete?

[lekoala]

@GuySartorelli you can leave this open, i have a few others coming up but I'm rather in a rush at the moment so that could take a few weeks