Update comments to reflect that memory cache isn't capped anymore.

A previous change https://github.com/square/picasso/commit/f79dd0af3274b511c6d9521aa9e5cf3b2ee7d704 removed the cap on the memory cache. Update the comments to reflect this change.
Merge pull request #323 from f2prateek/patch-1

Update comments to reflect that memory cache isn't capped anymore.