bytes: note that NewBuffer's initial size can change

bytes.NewBuffer's documentation says it can be used to set the initial
size of the buffer. The current wording is:

> It can also be used to size the internal buffer for writing.

This may led users to believe that the buffer (its backing array) is
fixed in size and won't grow, which isn't true (subsequent Write calls
will expand the backing array as needed).

Change the doc to make it clearer that NewBuffer just sets the initial
size of the buffer.

Fixes #27242

Change-Id: I2a8cb5bee02ca2c1657ef59e2cf1434c7a9bd397
Reviewed-on: https://go-review.googlesource.com/132035


Reviewed-by: Dominik Honnef <dominik@honnef.co>
Reviewed-by: Ian Lance Taylor <iant@golang.org>