PhpDoc: Replace non-existing param type mode with string
Summary: `mode` is not a valid type. See https://docs.phpdoc.org/guide/references/phpdoc/types.html
Test Plan: Read docs; run static code analysis.
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26305