Clarify the purpose of ObjectInserter.buffer()

Clarify the purpose of ObjectInserter.buffer()

Recently Robin tried to increase the size of the buffer used by
ObjectInserter to fix a bug in the InputStream that handles AutoCRLF.
The purpose of this buffer is NOT to make a random InputStream work
correctly by passing it a larger buffer during read(byte[],int,int).

Clarify the Javadoc on the buffer() method to reduce the risk
someone tries to abuse it again.

While we are here, modify the method to load the field into a local
variable before returning. This should cut down 1 field load during
the common case of the buffer being already allocated.

Change-Id: Ic6898530d10fcd7e59f90397117a4a0d97e1f031
Signed-off-by: Chris Aniszczyk <zx@twitter.com>