ffivector: call destructor during resize, add documentation

Summary:
I started by writing the documentation, and, as I got to explain what happens
during resize(), it dawned upon me that... well, nothing does.

Test Plan: ffivector_test

Reviewed By: tulloch@fb.com

FB internal diff: D1463447
ffivector: call destructor during resize, add documentation

Summary:
I started by writing the documentation, and, as I got to explain what happens
during resize(), it dawned upon me that... well, nothing does.

Test Plan: ffivector_test

Reviewed By: tulloch@fb.com

FB internal diff: D1463447