Make selecting multiple titles in advanced search easier

[sseymore]

From UX testing:

10. Search for “apples” in The Monmouth herald (Monmouth, Or.) and The Evening herald (Klamath Falls, Or.): There were many issues with this task due to the interface of selecting multiple newspapers. Users did not understand which keys to use to select multiple newspapers. Also, scrolling to select multiple newspapers caused users to forget/lose the first title they selected.

Recommendation: Make CMD/CTRL note clearer about which system uses which key and add more description about selecting multiple titles with this key. Possibly align this note with box of titles.