docs: add missing dirstate-v2 documentation
A user trying out the dirstate-v2 feature without the Rust extensions is
currently faced with an error message suggesting this help section, which does
not exist.
Differential Revision: https://phab.mercurial-scm.org/D11809
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true