documentation: metatable is a reference (refman is unclear)

[nobody <nobody+lua-list@...>]

Hey *,

(unless I fail at searching), the reference manual does not explicitly
state that metatables are "stored" by reference.  (I.e. `setmetatable`
doesn't copy the thing passed as metatable, but puts a reference to it
in a suitable location.)

The closest thing to information on this is the next-to-last paragraph
of §2.1:

> "Tables, [...] are objects: variables do not actually contain these
> values, only references to them. Assignment, parameter passing, and
> function returns always manipulate references to such values; these
> operations do not imply any kind of copy."

While it's clear from this that `setmetatable` receives a reference to
the (soon-to-be) metatable, `setmetatable` could do anything with that
(keep the reference, create a copy, …)  To someone who knows Lua, it's
clear that it just stores the reference (simplicity and all that), but
§2.4 (Metatables and Metamethods) completely avoids the topic, and the
description for `setmetatable` and `lua_setmetatable` just states that
they "set" the metatable.

Should this be clarified?  (Judging from a recent question on SO, this
has the potential to confuse some people coming from other languages.)

-- nobody