Docs: Clarify file I/O performance requirements

Clarify that the requirement is applicable to the internal storage,
used to store app private data (typically the data/ partition), and
that the provided number is the minimum number.

Also reduced the limit of the sequential write performance as per
http://android-review.googlesource.com/136980.

Bug: 19448302
Change-Id: I8ac6b3b02e254e2f457f63b52fae6495b0d8b769
1 file changed