Update the documentation for Mykomap's config paramter for searchableFields and other field-related parameters

[wu-lee]

Mm Version: 3.0.2+

The current documentation for this (and some other) config parameters doesn't explain what the fields used in the value can be, or how to find that out. We need to explain:

• what the core fields are (which every map has)
• how to find out what the custom fields are

Currently the latter isn't obtainable in a very user-friendly way, as the main source of information is in the config.ts file (or the published config.json file, which at least is possible to inspect via ShipShape; although again that's not super simple as it requires clicking on the right link and interpreting some JSON).

So, in the first instance, we can just explain for developers and technical people; for end users, there may need to be some new features requested.