Update language documentation

- Replace ?{}/!{} with ?[]/![].  The list syntax is appropriate for
  this, and we save { and } for possible block syntax at some point.

- Make [foo] not a literal form, meaning it doesn't preserve the
  stack.  This is to make list generators more similar to formatting
  strings.  Introduce +[foo] for the form that does preserve stack,
  which is what + ordinarily denotes.

- Drop special handling of DW_FORM_flag{,present} attributes.  This
  interpretation was suprising, and people would need to remember to
  write the actual ?@at like ?[attribute name DW_AT_at ?eq].  Note
  that this never was implemented anyway.

- Make if and else applicable to boolean values as well.  Introduce
  not, so that one doesn't need to write dangling else's when (not if)
  is what they have in mind.  Promote if/else/not to introduction to
  explain how to use them to determine whether DIE really is a
  declaration (now that ?@declaration just denotes attribute presence,
  it might still be false).

- Make find look for sub-sequences/sub-string in a sequence/string.

- Add value assertions to Overview section, drop Unsettled section.

- None of the above is implemented as of now.