Add documentation of cpp.toolchainInstallPath property
Add documentation of cpp.toolchainInstallPath property

The property is undocumented, but plays a central role in most toolchain
setups. Therefore it should be documented.

Fixes: QBS-1422
Change-Id: I9d1044733e6b99862f137849502e5b1a35a4c192
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@qt.io>
Reviewed-by: Leena Miettinen <885fda02d52616fec24ed5fed9b3f0568fce3ead@qt.io>

qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs