Update alltt.dtx to document the fact that the glyph at the position …

…f the $-sign differs in for different OT1-encoded fonts.

base/alltt.dtx

@@ -40,10 +40,11 @@
 %<package>\ProvidesPackage{alltt}
 %<driver>\ProvidesFile{alltt.drv}
 %\ProvidesFile{alltt.dtx}
-              [2020/11/23 v2.0g defines alltt environment]
+              [2021/01/29 v2.0g defines alltt environment]
 %
 %<*driver>
 \documentclass{ltxdoc}
+\usepackage{alltt}
 \begin{document}
 \providecommand{\Lenv}[1]{\textsf{#1}}
 \providecommand{\pkg}[1]{\texttt{#1}}
@@ -55,6 +56,8 @@
 %
 % \changes{v2.0g}{1997/06/16}{A few documentation fixes (PR 2520)}
 % \changes{v2.0d}{1995/04/02}{A few documentation fixes (PR 1517)}
+% \changes{v2.0g}{2021-01-29}{Added a warning about OT1 versus T1 font
+% encoding}
 %
 %  \GetFileInfo{alltt.dtx}
 %  \title{The \texttt{alltt} environment\thanks{This file
@@ -88,11 +91,19 @@
 %    extra blank line if you're not careful.
 %
 %   \item Insert a math formula.  Note that |$| just produces a dollar
-%   sign, so you'll have to type |\(...\)| or |\[...\]|.  Also, |^|
-%   and |_| just produce their characters; use |\sp| or |\sb| for
-%   super- and subscripts, as in |\(x\sp{2}\)|.
+%    sign, so you'll have to type |\(...\)| or |\[...\]|.  Also, |^|
+%    and |_| just produce their characters; use |\sp| or |\sb| for
+%    super- and subscripts, as in |\(x\sp{2}\)|.
 %   \end{itemize}
 %
+%    \textbf{NB} When you are using \textsf{OT1} encoded fonts you might
+%    be surprsied when you switch to italics, becuase those fonts have
+%    a different set of glyphs:
+%    \begin{alltt}
+%  The glyph at the position of the $ in a slanted font: \textsl{$}.
+%  The glyph at the position of the $ in an italic font: \textit{$}.
+%    \end{alltt}
+%
 %  \StopEventually{}
 %
 %  \section{The Implementation}