Tighten up description of an integer expression

The defintion given for \int_eval:n is referenced elsewhere, so the
other functiosn should not need this information so formally. The bit
about expansion and TeX assignments is really not 'expl3-level' so should
be in a TeXnote.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5421 de43f980-851b-0410-b2f7-c40aca1f87e0