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>

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