Document and check that the alphabet cannot contain '\n' or '\r'.
Document that the alphabet cannot contain the padding character.
Document that the padding character must be equal or bellow '\xff'.
Document that the padding character must not be '\n' or '\r'.

Fixes #19343
Fixes #19318

Change-Id: I6de0034d347ffdf317d7ea55d6fe38b01c2c4c03
Reviewed-on: https://go-review.googlesource.com/37838Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>