doc: remove outdated information about interactive.singleKey

doc: remove outdated information about interactive.singleKey

The Perl implementation of add --interactive was removed in commit [1].

Additionally, the interactive.singleKey setting is no longer silently
ignored. The internal implementation of ReadKey [2] displays a warning
if the platform is unsupported.

[1] 20b813d7d (add: remove "add.interactive.useBuiltin" & Perl "git add--interactive", 2023-02-06)
[2] a5e46e6b0 (terminal: add a new function to read a single keystroke, 2020-01-14)

Signed-off-by: Julio Bacellari <julio.bacel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>