Clarify and simplify documentation for the os module
Clarify and simplify documentation for the os module

I have done the following changes:

* Deleted the notes about null characters and `=` having been allowed
  but are now being rejected. That change happened three years ago.

* Documented that an environment variable name is allowed to contain
  `=` as the first character on Windows.

* Removed the vague warnings that "not all invalid characters
  necessarily will cause the primitive operations to fail, but may
  instead produce invalid results".

* Added a single note to note that the functions will fail with `badarg`
  if the arguments contains invalid characters.

vladdu/otp,dgud/otp,dumbbell/otp,uabboli/otp,potatosalad/otp,erlang/otp,emacsmirror/erlang,g-andrade/otp,getong/otp,mikpe/otp,isvilen/otp,bjorng/otp,g-andrade/otp,g-andrade/otp,getong/otp,getong/otp,dgud/otp,getong/otp,jj1bdx/otp,emacsmirror/erlang,vinoski/otp,jj1bdx/otp,dgud/otp,dumbbell/otp,erlang/otp,ferd/otp,vladdu/otp,rlipscombe/otp,bjorng/otp,dgud/otp,potatosalad/otp,rlipscombe/otp,vinoski/otp,dgud/otp,dgud/otp,emacsmirror/erlang,vladdu/otp,emacsmirror/erlang,vladdu/otp,mikpe/otp,isvilen/otp,uabboli/otp,g-andrade/otp,emacsmirror/erlang,vinoski/otp,isvilen/otp,vladdu/otp,jj1bdx/otp,dumbbell/otp,bjorng/otp,jj1bdx/otp,uabboli/otp,potatosalad/otp,mikpe/otp,mikpe/otp,isvilen/otp,erlang/otp,g-andrade/otp,mikpe/otp,potatosalad/otp,getong/otp,isvilen/otp,dgud/otp,erlang/otp,uabboli/otp,ferd/otp,emacsmirror/erlang,erlang/otp,mikpe/otp,ferd/otp,uabboli/otp,jj1bdx/otp,lrascao/otp,vinoski/otp,isvilen/otp,rlipscombe/otp,vinoski/otp,mikpe/otp,g-andrade/otp,uabboli/otp,getong/otp,isvilen/otp,bjorng/otp,emacsmirror/erlang,ferd/otp,dumbbell/otp,isvilen/otp,uabboli/otp,rlipscombe/otp,getong/otp,dumbbell/otp,ferd/otp,vladdu/otp,mikpe/otp,bjorng/otp,uabboli/otp,potatosalad/otp,vinoski/otp,vinoski/otp,vinoski/otp,jj1bdx/otp,isvilen/otp,jj1bdx/otp,jj1bdx/otp,uabboli/otp,emacsmirror/erlang,jj1bdx/otp,mikpe/otp,rlipscombe/otp,ferd/otp,ferd/otp,vinoski/otp,lrascao/otp,g-andrade/otp,jj1bdx/otp,g-andrade/otp,rlipscombe/otp,bjorng/otp,getong/otp,vladdu/otp,lrascao/otp,dgud/otp,bjorng/otp,getong/otp,g-andrade/otp,vladdu/otp,lrascao/otp,dumbbell/otp,emacsmirror/erlang,bjorng/otp,lrascao/otp,erlang/otp,bjorng/otp,bjorng/otp,dgud/otp,dgud/otp,rlipscombe/otp,rlipscombe/otp,dumbbell/otp,lrascao/otp,mikpe/otp,lrascao/otp,potatosalad/otp,erlang/otp,erlang/otp,dumbbell/otp,ferd/otp,vladdu/otp,erlang/otp,rlipscombe/otp,potatosalad/otp,potatosalad/otp,dumbbell/otp,lrascao/otp,erlang/otp,potatosalad/otp,emacsmirror/erlang,isvilen/otp,vinoski/otp,ferd/otp,getong/otp,lrascao/otp,g-andrade/otp,dumbbell/otp,potatosalad/otp,rlipscombe/otp