[Document] Need a detailed description about some methods

[Losses]

Hi, I'm the author of #266 ,

1. On the language options section, both stemmer, filter, matcher said we could pass a Function into it, but what does it looks like? Since I haven't used this feature, I was confused about how to write type annotation for it.

2. The document said that stemmer could be a custom object, filter and matcher could be a custom array, but this is not consistent with previous type definition,

flexsearch/index.d.ts

Lines 120 to 122 in 5dfb3de

	type Stemmer = { [key: string]: string };
	type Matcher = { [key: string]: string };
	type FilterFn = (str: string) => boolean;

and even not consistent with the following document:

matcher, type: {string: string}
stemmer, type: {string: string}
filter, type: string[]

are they changed on 0.7.0?

3. The second parameter of registerCharset and registerLanguage is not clear enough, they are Charset Options, Language Options or something else?

I've noticed that there are something called 'tokenize' from:

flexsearch/src/index.js

Line 82 in 04c7332

this.tokenize = tmp = (charset && charset.tokenize) || options["tokenize"] || "strict";

but this is not showed up on the Charset Options section, this confused me a lot.

Thanks for your time!

[mmm8955405]

I think the documentation should be updated. Otherwise, I don't know how to use this. I look confused