Improve documentation of addFilter in apachesolr.api.php

The apachesolr.api.php documentation for addFilter is confusing for newcomers. Any person reading the following example will automatically assume that the filter is referring to the English word "is":

function hook_apachesolr_query_alter($query) {
  // I only want to see articles by the admin!
  $query->addFilter("is_uid", 1);
}

...when in fact it means nothing of the sort :(

It is shorthand for Integer Single (abbreviated "is"), which is, to say the least, unintuitive for someone with no Solr experience. We need to add an explanation of this difference and provide instructions for the user to go inspect schema.xml so developers can properly learn how deep and complex the functionality of addFilter really is.