[3.14] gh-136752: Clarify documentation for ``IPv{N}Address.is_reserved`` (GH-136794) (#136827)

gh-136752: Clarify documentation for ``IPv{N}Address.is_reserved`` (GH-136794)
(cherry picked from commit 6293d8a1a648a498b7ac899631b74fa25c71c1ac)

Co-authored-by: Matthieu Lienart <50069805+mlnrt@users.noreply.github.com>
Co-authored-by: Matthieu Lienart <matthieu.lienart@axians.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>