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

### 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.