[Modules] Handle man pages search path

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

Dear All,

Following "Issue around MANPATH in Modules 5.5.0", I have added a
cookbook recipe in the documentation to describe what to do if your
modulefiles update MANPATH env var to retain access to system man
pages:

https://modules.readthedocs.io/en/latest/cookbook/man-path.html

Regards,
Xavier