Oxygen.com's Babynamer (a reasonably accurate site which is the exception to the rule of "avoid name sites with the word 'baby' in the title") also mentions the 13th-century citation (although they say the citation occurs in 1212, rather than 1221--this could be a transposition error) in their entry of
Amanda:
http://snipurl.com/8l3k.
I love that medieval names site, btw. It has Admiranda, which I theorise could be another way Shakespeare could've gotten
Miranda.
Miranda