Improve docs for buffer.toString and buffer.copy

Improve documentation for buffer.toString and buffer.copy per issues #8859 and #8857.
maybe change to "`undefined` or anything not parseable as a number", @trevnorris is this true for v0.10 and is it a constraint we should tighten for v0.12? I'm not sure I can think of a valid reason to let invalid input go "ignored".