Enhancements for php-search-(local-)documentation.

* In the local documentation directory, all of the following filename
  prefixes are now considered when looking for a match:
  "function" "control-structures" "class" "book"
  (previously, only "functions" was used).

* If there is no current word at point, or if a prefix argument is
  supplied, then prompt the user for a word to search for. If local
  documentation is available, then this is used to provide completions.

* New variable `php-search-documentation-browser-function` enables a
  browser to be configured specifically for viewing PHP documentation.
  The new built-in browser works particularly nicely with the local
  PHP documentation files (specify the `eww-browse-url` function).