A \colorbox adds some space before and after the text inside it; this space is \fboxsep wide; so
\parbox{\dimexpr\textwidth-2\fboxsep\relax}{#1#2#3}}
will do. You might also load the calc package and then write the same in the perhaps more natural syntax
\parbox{\textwidth-2\fboxsep}{#1#2#3}}
UPDATE (to reflect revised MWE):
The only thing I changed from your non-working MWE is that I added a \relax\strut\ignorespaces to the end of \argitem. If I am understanding it properly, the \item had to be ended, because you had no text as part of the item (the \parbox was part of the \item's optional argument). So any \strut applied immediately afterward was set back on the top line of the paragraph (as part of the \item). The \relax basically stopped the \strut from being gobbled by the \item. I'm probably wrong in this explanation, but someone will correct me.
\documentclass{article}
\usepackage{setspace,lipsum}
\makeatletter
\newcommand\singlelipsum[1]{%
\begingroup\let\lips@par\relax\csname lipsum@\@roman{#1}\endcsname
\endgroup }
\newlength{\argwidth}
\newcommand{\argparams}[1]{%
\parbox[t]{\argwidth}{%
\setstretch{1.5}%
#1)%
%\par\xdef\tpd{\the\prevdepth}% [1]
%\strut% [2]
}%
%\par\prevdepth\tpd% [1]
%\\\strut\ignorespaces% [2]
}
\newcommand{\argfunc}[1]{\textbf{#1}(}
\newcommand{\argitem}[2]{
\settowidth{\argwidth}{\argfunc{#1}}%
\addtolength{\argwidth}{-2\argwidth}%
\addtolength{\argwidth}{\textwidth}%
\item[\argfunc{#1}\argparams{#2}]\relax\strut\ignorespaces%
}
\newcommand{\argitemlabel}[1]{%
\@tempdima\linewidth%
\advance\@tempdima \leftmargin\makebox[\@tempdima][l]{#1}%
}
\newenvironment{arglist}{
\begin{list}{}{%
\setlength\labelwidth{\leftmargin}
\setlength\labelsep{0pt}
\setlength\rightmargin{0pt}
\let\makelabel=\argitemlabel%
}%
}{\end{list}}
\makeatother
\begin{document}
\setstretch{2}
\begin{arglist}
\argitem{function}{args} body
\argitem{function}{\singlelipsum{4}} \singlelipsum{2}
\end{arglist}
\end{document}
ORIGINAL SOLUTION (for original MWE):
I see two things going on. The first, concerning the bottom of a \parbox can be resolved with judicious use of \struts.
The second issue relates to the \lipsum[] material commonly used in MWEs. When lipsum paragraphs are output, they contain more than text. They contain paragraph formatting, in the form of \pars and stuff like that. Typically, if one were typing text as an argument for \arrowed, one would not end on a \par. And so, it would be better to get lipsum to output just the text of a paragraph, without the ending \par and such.
The author of lipsum told me how to do that:
\newcommand\singlelipsum[1]{%
\begingroup\let\lips@par\relax\csname lipsum@\@roman{#1}\endcsname
\endgroup }
And so, with the combination of \strut and \singlelipsum (which gives just the text of a single lipsum paragraph), one can fix the problem with
\documentclass{article}
\usepackage{lipsum}
\usepackage{setspace}
\setstretch{1.5}
\setlength{\parindent}{0pt}
\makeatletter
\newcommand\singlelipsum[1]{%
\begingroup\let\lips@par\relax\csname lipsum@\@roman{#1}\endcsname
\endgroup }
\makeatother
\newlength{\len}
\newcommand{\arrowed}[1]{%
% draw an arrow, then a parbox that's exactly
% the remaining width of the line
\settowidth{\len}{$\rightarrow$}%
\addtolength{\len}{-2\len}%
\addtolength{\len}{\textwidth}%
$\rightarrow$\parbox[t]{\len}{#1\strut}\\\strut\ignorespaces%
}
\begin{document}
\lipsum[2]
\arrowed{\singlelipsum{4}}
\lipsum[6]
\arrowed{But this is OK, since it's only one line.}
\lipsum[2]
\end{document}
Best Answer
The space between the box edges and the real content is ruled by
\fboxsepas for the usual\fboxcommand.Since
\fboxsep4mmappears explicitly in the\informationmacro definition there's no chance to change the\fboxsepvalue before.4mmseems to be definitely too much, but this is a matter of taste, of course.In my point of view, there's no need of using an additional
\frameboxcommand, since\fcolorboxis also available.