docs: add inline documentation

[iShibi]

I have added docs only for the four major functions. Once the writing style gets a green flag I would be happy to document all the other stuff.

[RafaelGSS]

I believe the inline documentation is useful just for the methods exported by this library. Internal methods should have their naming clear enough

[iShibi]

By other stuff I meant the interfaces. Doing so would allow the editor to show what a particular field in an object parameter does.

[RafaelGSS]

Including SearchResult would be great, yeah.

[RafaelGSS]

Most of the inline documentation is pretty redundant to me. Example:

export interface Lyra<S extends PropertiesSchema> extends Data<S> {
  /**
   * The default language analyzer in use.
   */
  defaultLanguage: Language;
  /**
   * The structure of documents stored in the database.
   */
  schema: S;
  edge: boolean;
}

You don't need to add inline documentation to everything, just mention unclear fields.

For instance:

  /**
   * The maximum [levenshtein distance](https://en.wikipedia.org/wiki/Levenshtein_distance)
   * between the term and the searchable property.
   */
  tolerance?: number;

is a great addition. As said previously, I'd keep just the inline doc for exported functions + unclear fields (tolerance)