git-reset.txt: make modes description more consistent

Currently, the structure of the individual mode entries is different
which makes it difficult to grasp the differences between the modes.
Also, the same items are named differently (e.g. <commit>, "the named
commit", "the given commit", "the commit being switched to").

Structure and word all mode entries consistently.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-reset.txt

@@ -15,13 +15,13 @@ SYNOPSIS
 DESCRIPTION
 -----------
 In the first and second form, copy entries from <commit> to the index.
-In the third form, set the current branch head to <commit>, optionally
+In the third form, set the current branch head (HEAD) to <commit>, optionally
 modifying index and working tree to match.  The <commit> defaults to HEAD
 in all forms.
 
 'git reset' [-q] [<commit>] [--] <paths>...::
 	This form resets the index entries for all <paths> to their
-	state at the <commit>.  (It does not affect the working tree, nor
+	state at <commit>.  (It does not affect the working tree, nor
 	the current branch.)
 +
 This means that `git reset <paths>` is the opposite of `git add
@@ -43,9 +43,10 @@ This means that `git reset -p` is the opposite of `git add -p` (see
 linkgit:git-add[1]).
 
 'git reset' [--<mode>] [<commit>]::
-	This form resets the current branch head to <commit> and then
-	updates index and working tree according to <mode>, which must
-	be one of the following:
+	This form resets the current branch head to <commit> and
+	possibly updates the index (resetting it to the tree of <commit>) and
+	the working tree depending on <mode>, which
+	must be one of the following:
 +
 --
 --soft::
@@ -60,22 +61,30 @@ linkgit:git-add[1]).
 	been updated. This is the default action.
 
 --hard::
-	Matches the working tree and index to that of the tree being
-	switched to. Any changes to tracked files in the working tree
-	since <commit> are lost.
+	Resets the index and working tree. Any changes to tracked files in the
+	working tree since <commit> are discarded.
 
 --merge::
-	Resets the index to match the tree recorded by the named commit,
-	and updates the files that are different between the named commit
-	and the current commit in the working tree.
+	Resets the index and 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 (i.e. which have changes
+	which have not been added).
+	If a file that is different between <commit> and the index has unstaged
+	changes, reset is aborted.
++
+In other words, --merge does something like a 'git read-tree -u -m <commit>',
+but carries forward unmerged index entries.
 
 --keep::
-	Reset the index to the given commit, keeping local changes in
-	the working tree since the current commit, while updating
-	working tree files without local changes to what appears in
-	the given commit.  If a file that is different between the
-	current commit and the given commit has local changes, reset
-	is aborted.
+	Resets the index, updates files in the working tree that are
+	different between <commit> and HEAD, but keeps those
+	which are different between HEAD and the working tree (i.e.
+	which have local changes).
+	If a file that is different between <commit> and HEAD has local changes,
+	reset is aborted.
++
+In other words, --keep does a 2-way merge between <commit> and HEAD followed by
+'git reset --mixed <commit>'.
 --
 
 If you want to undo a commit other than the latest on a branch,