Expand “Containers versus values” trap

[AlexDaniel]

See this discussion: https://irclog.perlgeek.de/perl6/2017-02-24#i_14160027

We already have something here: https://docs.perl6.org/language/traps#Containers_versus_values_in_a_Capture

But let's add @a.push: ($a, $b) there as well, because on the first sight it looks very innocent indeed.

Also, the section talks about Captures, but I'm not sure what this is about. ($a, ++$a) itself is a List, so where's a capture in the given example?

[zoffixznet]

Can be useful to mention this on https://docs.perl6.org/language/containers instead/too.

[JJ]

@AlexDaniel if it's a List, it's been created through calling its initialization routine, so it becomes a capture somewhere, right?