doc: git-reset: clarify intro

>From user feedback, there were several points of confusion:

- What "tree-ish", "entries", "working tree", "HEAD", and "index" mean
  ("I have no clue what the index is", "I've been using git for 20 years
  and still don't know what a tree-ish is"). Avoid using these terms
  where it makes sense.
- What "optionally modifying index and working tree to match" means
  ("to match what?" "optionally based on what?")
  Remove this from the intro, we can say it later when giving more
  details.
- One user suggested that "The <tree-ish>/<commit> defaults to HEAD
  in all forms." should be repeated later on, since it's easy to miss.
  Instead say that HEAD is the default in each case later.

Another issue is that `git reset` consistently describes the action
it does as "Reset ...", commands should not use their name to describe
themselves, and that the word "mode" is used to mean several different
things on this page.

Address these by being more clear about two use cases for `git reset`
("to undo operations" and "to update staged files"), and explaining what
the conditions are for each case instead of forcing the user to figure
out the pattern is in first form vs the other 3 forms.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jvns

Documentation/git-reset.adoc

@@ -3,7 +3,7 @@ git-reset(1)
 
 NAME
 ----
-git-reset - Reset current HEAD to the specified state
+git-reset - Set HEAD to point at the specified commit
 
 SYNOPSIS
 --------
@@ -15,10 +15,13 @@ git reset (--patch | -p) [<tree-ish>] [--] [<pathspec>...]
 
 DESCRIPTION
 -----------
-In the first form, set the current branch head (`HEAD`) to _<commit>_,
-optionally modifying index and working tree to match.
-The _<tree-ish>_/_<commit>_ defaults to `HEAD` in all forms.
-In the last three forms, copy entries from _<tree-ish>_ to the index.
+`git reset [<mode>] <commit>` changes which commit HEAD points to.
+This makes it possible to undo various Git operations, for example
+commit, merge, rebase, and pull.
+
+However, when you specify files or directories or pass `--patch`,
+`git reset` will instead update the staged version of the specified
+files without updating HEAD.
 
 `git reset [<mode>] [<commit>]`::
 	This form resets the current branch head to _<commit>_ and