Fixed reference manual for RM 26

Removed entries w.r.t periodic in VDM++ as those features have been
removed since release 2.1.2. Impact on Chapter 15 and Appendix A.4.8.
Merge branch 'master' of https://github.com/overturetool/language

Conflicts:
	documentation/VDM10LangMan/VDM10_lang_man.pdf
Fixed commit conflict