[Modules] Shell support

[Xavier D. <xav...@gm...>]

Hello All,

I have just added a "Shell support" section in modulefile man page to
describe what shells are supported by Modules and more specifically what
kind of environment changes are supported by these shells (variable,
function changes, ...):

https://modules.readthedocs.io/en/latest/modulefile.html#shell-support

Regards,
Xavier