[enhancement][docs] Document impl in DSLX reference docs

[cdleary]

What's hard to do? (limit 100 words)

DSLX got a new impl keyword/facility! 🎉 I was searching for it in the DSLX reference docs but don't think it's added there yet. https://google.github.io/xls/dslx_reference/ This bug is to track its addition to the docs if it's ready for use/testing.

cc @richmckeever

Current best alternative workaround (limit 100 words)

Reading the source (for developers) or pattern matching existing samples in the language tests.

Your view of the "best case XLS enhancement" (limit 100 words)

Design intent / philosophy, current working status, known limitations in DSLX reference.