Fixing old documentation regarding PHP code examples

[zaerl]

Motivation for the change, related issues

Context: #2686

Documentation should be updated to reflect the new change, where the root folder has been moved from / to /wordpress and not a relative path. So all the old require_once 'wordpress/wp-load' should now be documented as a require_once '/wordpress/wp-load' to be sure not having a warning in the console.

Implementation details

Fixed some comments and documentation

Testing Instructions (or ideally a Blueprint)

Nothing to test in particular, a quick check at the code is enough.

[adamziel]

Let's spell out what this PR is doing in the title and in the description, it will help the future us browse the commit history without additional clicks

[zaerl]

Let's spell out what this PR is doing in the title and in the description, it will help the future us browse the commit history without additional clicks

Done, thanks.

[adamziel]

Thank you!