Document capture list for BOOST_COMPUTE_CLOSURE()

Document the capture list for `BOOST_COMPUTE_CLOSURE()` better. Indicate that it is a Boost.PP tuple rather than a sequence (perhaps have an example showing capturing multiple values).

From the review:

```
Also,
doesn't explain what format the lists (arguments and capture) should be
in. Preprocessor functions often take lists in other formats, such as a
Boost.PP Sequence
,
and your example only has one-element lists, so it's not enough to
clarify this point. (To be clear: I'm not suggesting you should use
Boost.PP sequences, merely that you should make it clear in the docs
whether you are).
```
##