List unpacking

It is quite often happens that you know the size of a list and you would want to give its elements names instead of indexing them. Something like:
```jam
local name version = [ MATCH "^(\w+)-(.*)" : $(toolset) ] ;
```
However, that syntax is already taken by multiple assignment operation, so in example above both variables will contain the same value. Is it documented? I had no luck finding anything in documentation about it. May be there is list unpacking already too? Or may be we can repurposed the syntax for it?
I grepped Boost and only found the use in some B2 tests:
```bash
>grep --include "[Jj]am[fr]*" --include "*.jam" -PR "^\s*(local\s+)?+(\w+\s+){2,}=" .
./tools/build/src/build/feature.jam: local p fullp = $(x[1]) ;
./tools/build/test/core-language/test.jam: local v1 v2 = d e f ;
./tools/build/test/core-language/test.jam: local v1 v1 = d e f ;
```

Review the multiple-assignment examples in tools/build/test/core-language/test.jam and the related assignment use in tools/build/src/build/feature.jam. Determine whether list unpacking is documented or intended, then make the behavior or documentation explicit and add or adjust a core-language test for the chosen outcome.

Written by the indexing model from the issue text.