Document our new alter_table command

[macintux]

This change also correct some minor inaccuracies with the current text on changing tables.

Requires https://github.com/basho/riak_shell/pull/75 in particular and several related PRs yet to be approved.

[paegun]

+1

There will be a mention of TTL bucket-type property when that becomes available, so +1. The TTL property will be added w/i the table of properties https://github.com/basho/basho_docs/pull/2411/files#diff-e83cb88e242cff57b077a249eb90fc3aR259

[macintux]

@paegun would appreciate another +1 if you're ok with my most recent commit

[hazen]

+1

[paegun]

+1

[lrnrthr]

👍 🌵