Further clarification on how the \ior_get(_str):NN functions work

Document situation with tabs and line end conversion.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4589 de43f980-851b-0410-b2f7-c40aca1f87e0
Some more clarification of \ior_get_str:NN 

Testing shows tabs/spaces are both catcode 10 when read this way. Copy in
also some text from Chris.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4590 de43f980-851b-0410-b2f7-c40aca1f87e0