Two documentation suggestions

[Aman Verma <amanraoverma+vim@...>]

First, I just wanted to say how impressed I am at the remarkable brevity
of the Lua reference manual. In reading it, I have come across a couple
of issues that I think could be fixed:

One
===
The description of the second parameter to `os.date` is this:

> If the time argument is present, this is the time to be formatted (see
> the os.time function for a description of this value). Otherwise, date
> formats the current time.

I think it would be clearer if it specified that the reader should look
at the _return value_ of `os.time`. Otherwise, it is unclear which part of
the description of `os.time` the reader should look at: the return value
or the parameter.

Also, I'm not sure what the policy is for updating reference manuals for
unsupported versions of Lua, but this oversight exists in all the
manuals since 5.1.

Two
===
The documentation for `io.open` mentions opening a file in binary mode:

> The mode string can also have a 'b' at the end, which is needed in some
> systems to open the file in binary mode.

Perhaps this explanation is enough for someone who knows C. But as
someone who is simply a Lua user (not an embedder), this sentence is
vague. Is it possible to add more detail or is this one of the
implementation-defined things?

Best regards,
Aman