Better specify the requirements the parent class needs to have.
KConfigCompiler generates different variants of ctors taking either:
- a QStringLiteral argument (name set in <kcfgfile>
- a KSharedConfig::Ptr argument (arg="true" in <kcfgfile>)
- no argument (Inherits=true in kcfgc and no <kcfgfile>)
In order to have Inherits generate compiling code in all cases the
parent class needs to provide accessible ctors.
This change updates the docuementation to reflect this.