Metasploit-framework: Documentation on command stagers out of date

[ejholmes]

The documentation about command stagers at https://github.com/rapid7/metasploit-framework/wiki/How-to-use-command-stagers is out of date.

Primarily:

1. The links to the different flavors are broken, and should point to rex-exploitation
2. Information about the wget and curl flavors is missing.

[wvu-r7]

I should update it.

[gwillcox-r7]

@ejholmes Links should now be updated with @ccondon-r7's recent commit and one update I made just now. The wget and curl flavor documentation is however still missing.

[wvu-r7]

@gwillcox-r7 should update it.