BreakingChanges: document that we do not plan to deprecate git-checkout

BreakingChanges: document that we do not plan to deprecate git-checkout

The git-checkout(1) command is seen by many as hard to understand
because it connects two somewhat unrelated features: switching between
branches and restoring worktree files from arbitrary revisions. In 2019,
we thus implemented two new commands git-switch(1) and git-restore(1) to
split out these separate concerns into standalone functions.

This "replacement" of git-checkout(1) has repeatedly triggered concerns
for our userbase that git-checkout(1) will eventually go away. This is
not the case though: the use of that command is still widespread, and it
is not expected that this will change anytime soon.

Document that all three commands will remain for the foreseeable future.
This decision may be revisited in case we ever figure out that most
everyone has given up on any of the commands.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>