git reset --merge documentation improvments

Hello,

The current documentation for --merge is:

"Resets the index [1] and updates the files in the working tree that
are different between <commit> and HEAD, but keeps those which are
different between the index [2] and working tree"

I think this is confusing, because [1] is the *after-reset* index and
[2] is the *before-reset* index. If you fail to realise this it looks
like this "resets index (so index is empty) and then updates worktree
but skip files staged from index" --> "but there's nothing staged, the
index was reset!"

I think a better sentence would be:

"Updates the files in the working tree that are different between
<commit> and HEAD, but keeps those which are different between the
index and working tree, and finally resets the index."

Or something along those lines.

Philippe