Add a mechanism to include additional document in the generated JSDocs for Vim script functions

[mityu]

Is your feature request related to a problem?

Related to #279.

In function module, we cannot write additional notes in the JSDocs generated from Vim/Neovim's documents because they'll be all replaced by the document generator (deno-denops-std/.scripts/gen-functions/gen-functions.ts).

Since such additional document sometimes beneficial, e.g., write a note about better helper function, difference in behavior between Vim's builtin functions and denops-std's ones or etc, it must be useful to add a mechanism to include additional document in the generated JSDocs for functions under deno-denops-std/function/*.

Describe the solution you'd like

Introduce a new marker or something to indicate the following statement/paragraph is an additional note so that it won't be removed through automatic JSDoc generations.

Additional context