Update "Migration to version 12.0" wiki for reports

[dennybiasiolli]

In Odoo 12.0 the function for getting report values was renamed from get_report_values to _get_report_values.
It could be useful to specify this in the wiki section, task to do in the migration.

Something like:

• Rename all def get_report_values functions to def _get_report_values inside AbstractModel for reports

Thanks!

[pedrobaeza]

Well, that wiki tries to expose general changes, not as specific as this one, as there are tons of changes similar to this one depending on the area you are developing on, so the list can be very long. Anyway, I let others to judge if they consider that this should be added, being in the grey area.

cc @sbidoul @dreispt @simahawk

[dreispt]

I see this useful, but it is quite topic specific.
What I think it could work well is have a first document focusing only on the framework-wide changes, and another one highlighting specific item per App/Module.
That second document would mention specific changes to take care for when migration Accounting, Project, etc.

AFAICT this particular change is not important for reports, in general, but for some advanced cases, and probably the report-focused repos. So I >could see a Reports section in this second document.

Just my 2 cents.