kconfig_compiler documentation: Warn about calling save() in response to config notify signals.

We discovered (graphics/okular!305 (comment 126559)) that calling KConfigSkeleton::save() in response to a KConfigXT notify signal causes trouble. This adds a warning to the documentation.

When save() is called in a slot that listens directly to any KConfigXT notify signal,
the application will enter an infinite loop,
because the generated usrSave() method is not reentrant.
Config notify signals are emitted before their changed flag is cleared.