Define & Document a function name prefix for core use in plugins

i.e. we have `journal_entry()` now and a proposed `journal_entry_cqc()` (see #1030 ). But what if a plugin developer already has that function name in their code, with matching number of arguments?

We need to define a prefix for function names that plugin developers know not to use, and then always use it ourselves for any new API function names.

Start by reviewing the existing plugin-facing functions, including journal_entry() and the proposed journal_entry_cqc() from issue #1030. Define a reserved prefix that plugin developers are told not to use, document the convention, and identify how future core API names will follow it.

Written by the indexing model from the issue text.