bytes: document that buffer is reused in Truncate and Reset

Fixes #13671

Change-Id: Ic752de6a3030ff25474717505fa05895054217e7
Reviewed-on: https://go-review.googlesource.com/18029Reviewed-by: Russ Cox <rsc@golang.org>

src/bytes/buffer.go

@@ -36,10 +36,11 @@ const (
 // ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.
 var ErrTooLarge = errors.New("bytes.Buffer: too large")
 
-// Bytes returns a slice of the contents of the unread portion of the buffer;
-// len(b.Bytes()) == b.Len().  If the caller changes the contents of the
-// returned slice, the contents of the buffer will change provided there
-// are no intervening method calls on the Buffer.
+// Bytes returns a slice of length b.Len() holding the unread portion of the buffer.
+// The slice is valid for use only until the next buffer modification (that is,
+// only until the next call to a method like Read, Write, Reset, or Truncate).
+// The slice aliases the buffer content at least until the next buffer modification,
+// so immediate changes to the slice will affect the result of future reads.
 func (b *Buffer) Bytes() []byte { return b.buf[b.off:] }
 
 // String returns the contents of the unread portion of the buffer
@@ -60,7 +61,8 @@ func (b *Buffer) Len() int { return len(b.buf) - b.off }
 // total space allocated for the buffer's data.
 func (b *Buffer) Cap() int { return cap(b.buf) }
 
-// Truncate discards all but the first n unread bytes from the buffer.
+// Truncate discards all but the first n unread bytes from the buffer
+// but continues to use the same allocated storage.
 // It panics if n is negative or greater than the length of the buffer.
 func (b *Buffer) Truncate(n int) {
 	b.lastRead = opInvalid
@@ -74,8 +76,9 @@ func (b *Buffer) Truncate(n int) {
 	b.buf = b.buf[0 : b.off+n]
 }
 
-// Reset resets the buffer so it has no content.
-// b.Reset() is the same as b.Truncate(0).
+// Reset resets the buffer to be empty,
+// but it retains the underlying storage for use by future writes.
+// Reset is the same as Truncate(0).
 func (b *Buffer) Reset() { b.Truncate(0) }
 
 // grow grows the buffer to guarantee space for n more bytes.