Documenting Octavian behavior with @threads

[jlchan]

It seems from https://discourse.julialang.org/t/odd-benchmarktools-timings-using-threads-and-octavian/59838/5 that matmul! doesn't play well with @threads. Would it be useful to have this in the docs, maybe just as a warning or !!! note?

I'm happy to add this if it seems useful.

[chriselrod]

Yeah, it's worth noting.

[jlchan]

Addressed in #80