cmd/doc: truncate long lists of arguments

cmd/doc: truncate long lists of arguments

Some field-lists (especially in generated code) can be excessively long.
In the one-line printout, it does not make sense to print all elements
of the list if line-wrapping causes the "one-line" to become multi-line.

// Before:
var LongLine = newLongLine("someArgument1", "someArgument2", "someArgument3", "someArgument4", "someArgument5", "someArgument6", "someArgument7", "someArgument8")

// After:
var LongLine = newLongLine("someArgument1", "someArgument2", "someArgument3", "someArgument4", ...)

Change-Id: I4bbbe2dbd1d7be9f02d63431d213088c3dee332c
Reviewed-on: https://go-review.googlesource.com/36031
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>