Review and update the call-ins chapter

While researching another issue, I was going through the call-ins chapter and noticed several inconsistencies with how things work now - for example, there are lots of references to GTMCI (which in GT.M is the name of the internal "program" that drives call-ins much like GTMDMOD does for yottadb/mumps invocations). The issue is GTMCI was removed from the system permanently in one of the earliest YottaDB projects. There are others as well. The GTMCI references are in the "Nested Call-ins" chapter.

This issue is a placeholder that this review/update needs to be done.