The doc for `insert-char' is unclear what values are valid for CHARACTER
when called from Lisp. This clarifies that only code points can be
passed when called from Lisp instead of interactively.

> Interactively, prompt for CHARACTER using `read-char-by-name'.
> You can specify CHARACTER in one of these ways:

This reads to me as two separate, unrelated statements. Somemone could
interpret the latter as applying when called from Lisp, when in fact it's
only true when called interactively.

> Interactively, prompt for CHARACTER using `read-char-by-name'.
> When prompted, you can specify CHARACTER in one of these ways:

This is much better as it is now clear it only applies when
called interactively.

In addition, I added details on how to call `insert-char' from Lisp.

> When called from Lisp, you can only pass CHARACTER as a code point.
> See `char-from-name' for retrieving a code point from a Unicode character name.