bufio: document the relationship between UnreadByte and Peek

[bcmills]

#18556 disallowed calls to (*bufio.Reader).UnreadByte after any Peek, but did not update the documentation for UnreadByte, which currently reads:

UnreadByte unreads the last byte. Only the most recently read byte can be unread.

It is not obvious that Peek changes the meaning of (or invalidates) “the most recently read byte”, so the documentation for UnreadByte should be clarified.

(Or, better still, we should fix UnreadByte so that it works even after a Peek.)

[gopherbot]

Change https://golang.org/cl/155930 mentions this issue: bufio: document relationship between UnreadByte/UnreadRune and Peek