read-tree.txt: clarify --reset and worktree changes

read-tree.txt: clarify --reset and worktree changes

The description of --reset stays true to the first implementation in
438195cced (git-read-tree: add "--reset" flag, 2005-06-09). That is,
--reset discards unmerged entries. Or at least true to the commit
message because I can't be sure about read-tree's behavior regarding
local changes.

But in fcc387db9b (read-tree -m -u: do not overwrite or remove untracked
working tree files., 2006-05-17), it is clear that "-m -u" tries to keep
local changes, while --reset is singled out and will keep overwriting
worktree files. It's not stated in the commit message, but it's obvious
from the patch.

I went this far back not because I had a lot of free time, but because I
did not trust my reading of unpack-trees.c code. So far I think the
related changes in history agree with my understanding of the current
code, that "--reset" loses local changes.

This behavior is not mentioned in git-read-tree.txt, even though
old-timers probably can just guess it based on the "reset" name. Update
git-read-tree.txt about this.

Side note. There's another change regarding --reset that is not
obviously about local changes, b018ff6085 (unpack-trees: fix "read-tree
-u --reset A B" with conflicted index, 2012-12-29). But I'm pretty sure
this is about the first function of --reset, to discard unmerged entries
correctly.

PS. The patch changes one more line than necessary because the first
line uses spaces instead of tab.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>