The below docs are from pre-Qt 5.14 times and are now very out of date.
Starting with Qt 6.6 all this is no longer relevant - the QRhi APIs are now semi-public, with their entire documentation being part of the regular Qt documentation set.
See for example the QtGui module overview.

qdoc-generated documentation for QRhi. Currently matches patch set 73 of https://codereview.qt-project.org/c/qt/qtbase/+/256713 with some upcoming compute shader related work on top.

qdoc-generated documentation for the shadertools module