Document hidden param for multiple addresses

[micahalcorn]

This updates the parameters documentation to include num_addresses. The prior instruction caused some confusion here.

[emkman]

Agree this update is much needed as people are being limited by misleading docs when the code has great new functionality in it. You should also remove this note in the paragraph preceeding your change:

Currently, the HDWalletProvider manages only one address at a time, but it can be easily upgraded to manage (i.e., "unlock") multiple addresses.

Thanks!

[micahalcorn]

Good call @emkman 👍 I've added a commit.

[benjamincburns]

Thanks for this @micahalcorn! We've already included this via another PR.