[3.10] GH-101111: Disambigaute origin of const for store_const and append_const (GH-101121) (#101204)

(cherry picked from commit 9a155138c58cad409e28e34359ba87ec0025b6b7)

While the documentation for `optparse` mentioned that both `store_const` and
`append_const` store a constant value, it was not clear where this value was
coming from.

A link to `Option.const` makes this explicit.

Co-authored-by: Jürgen Gmach <juergen.gmach@googlemail.com>