Document treatment of trailing spaces by \tl_rescan:nn, etc. (closes #154)

The e-TeX \scantokens primitive treats its argument as a file, and so spaces
at the end are dropped. Adding a marker token at the end of the input can be
used to avoid this but at the cost that this token cannot appear in the real
input. As such, that is not a general fix and instead it's best to document
what happens here.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4647 de43f980-851b-0410-b2f7-c40aca1f87e0
Register mhchem prefix


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4648 de43f980-851b-0410-b2f7-c40aca1f87e0
Prefix change: with => withargs


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4649 de43f980-851b-0410-b2f7-c40aca1f87e0
Register prefix pkgloader


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4650 de43f980-851b-0410-b2f7-c40aca1f87e0
Register prefix graph


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4651 de43f980-851b-0410-b2f7-c40aca1f87e0