3.0: Multiple search issues

## Describe the bug
1. The task `node app @apostrophecms/search:index` fails with `TypeError: self.indexDoc is not a function`
2. Pieces types are not registered programmatically as stated [in the source documentation](https://github.com/apostrophecms/apostrophe/blob/3.0/modules/%40apostrophecms/search/index.js#L37) even with explicit `searchable: true` settings.
3. I'm unable to include a desired field in the search index even via explicit `searchable: true` field option (tested with `string` textarea field type).
4. The implementation of `addSearchTexts` disallows any weight configuration, and there are valid cases when a specific area text field might be of type `highSearchText`. I haven't tested if the rich-text widgets are searchable yet, and the above is obviously not a bug.
5. The `silent` option for fields is not documented anywhere and it's unclear how it would affect the search. It's definitely not a part of the search result UI, my guess is it has something to do with API result set? (again not a search related bug report)

## Expected behavior
1. It looks to be a typo. `indexDoc` is defined in the `handlers` configuration and not in `methods`.
2. I'm unable to find a registered handler for the `determineTypes` event in the pieces source but only dedicated method `self.searchDetermineTypes(types)` which in turn is not invoked anywhere (but I might miss something here). The only way to enable a piece for a search is via the explicit `types` option of the search module.
3. There is a trace to this feature in the `schema` module (`indexFields` method) and `string` field type has a dedicated `index` method.

## Details

**Apostrophe*
v3.0.1*

Start with the `node app @apostrophecms/search:index` entry point and the search module source at `modules/@apostrophecms/search/index.js`. Trace `indexDoc` in the handlers and methods configuration, then inspect `determineTypes`, `schema.indexFields`, and the string field's `index` method. Done means the reported indexing, type registration, field inclusion, and option documentation issues are resolved or clearly scoped.

Written by the indexing model from the issue text.