[vector] Fixes to documentation of into/fromByteBuffer

[Paul Sandoz]

Hi,

Here are changes to fix issues in the documentation of to/fromByteBuffer:

  http://cr.openjdk.java.net/~psandoz/panama/vector-fromByteBuffer-spec/webrev/ <http://cr.openjdk.java.net/~psandoz/panama/vector-fromByteBuffer-spec/webrev/>

This is based off the prior patch for cleanups [1].

Thanks,
Paul.

[1] http://cr.openjdk.java.net/~psandoz/panama/vector-cleanups/webrev/ <http://cr.openjdk.java.net/~psandoz/panama/vector-cleanups/webrev/>