Improvements to "Terms Filter" documentation

* Noted that setting the execution mode to "bool" works as as
  the "should" clause of a "bool filter", and not like the "must" or
  "must_not" clause.

* Removed the paragraph under the "Caching" heading, which incorrectly
  claimed that the "result of the filter is automatically cached by
  default".

* Moved the paragraphs about the `_nocache` options and the the
  `_cache` option into the "Caching" heading.

* Grammar and syntax cleanup.

Further questions I was unable to answer:

* For a "plain" query what is meant by " Iterates over all the terms,
  building a bit set matching it, and filtering."? Does this behave
  like an "or" query, like an "and" query, or something different? If
  it is different, how is it different?

* What is the difference between a "bool" query and an "or" query? The
  seem to have the same results, as far as I can tell. Why is "bool"
  mode preferred?

* Finally, the style guide is a little inconsistent. Line 20 uses the
  phrase "@terms@ filter", with "terms" marked as code, but in other
  places "and filter" and "or filter" are not marked as code. Which
  style is preferred?