Introduce `self` in `$scripting/#methods`

Description

I suggest we introduce the self explicitly in $scripting/#methods.
Specifically, explain how to interpret self appeared in reference pages, like this one:

The error message can be improved as well, but I don't know if that's possible.

Case 1: outline.entry.indented

#show outline.entry: it => {
  outline.entry.indented(
    prefix: it.prefix(),
    inner: it.body(),
    gap: 12em,
  )
}
#outline()

Today a poor guy writes the above code, and receives a misleading error message:

Missing argument: self

Noticing prefix and inner are positional, he/she removes the parameter names, and get another nonsense:

Expected outline entry

The correct usage is as follows (already included in the docs, but was ignored because it's far above):

#show outline.entry: it => {
  it.indented(
    it.prefix(),
    it.body(),
    gap: 12em,
  )
}

Another working solution:

#show outline.entry: it => {
  outline.entry.indented(
    it,
    it.prefix(),
    it.body(),
    gap: 12em,
  )
}

Case 2: selector.or

Many people don't know selector.or can be used as a plain function.

#selector(a).or(b, c) // okay
#selector.or(a, b, c) // also okay, but not everyone knows

Start with the $scripting/#methods documentation and compare the outline.entry.indented and selector.or reference examples. Explain how self is interpreted in reference pages and make the method and plain-function forms understandable; consider the reported error-message improvement separately because the issue does not establish whether it is feasible.