USABILITY: search syntax help link

Few users know the power and flexibility of the search syntax. The search.module currently supports an implied AND between words with an optional, phrases and exclusionary terms. I propose two changes:

1. Add a help link to to the search page.
2. As an adventurous user, I tried using lower case "or" and when this didn't work, I didn't try it again. We could improve this user experience by either (a) supporting case-insensitive OR, or (b) add a warning message to the user when one of the terms is lower case 'or'. Google does the latter, using the message "Try uppercase 'OR' to search for either of two terms." However, Google also searches for or=OR=Oregon. Since we exclude terms that are 3 characters or less (this is configurable, but 3 characters is the default), I think that option (a) is better for Drupal.