doc: update email search cheatsheet
doc: update email search cheatsheet

- Note that `subject:(a b)` is effectively an OR search (although AFAICT
  this contradicts the documentation even though it matches what I
  observe to happen in reality).
- Clarify that regex patterns containing spaces must be quoted.
- Drop `folder:` example because it doesn't seem to work, even after a
  full reindex and checking things with `NOTMUCH_DEBUG_QUERY=1 notmuch
  search ...`.
- Expand `is:...` examples.
- Add `id:...`.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent