Add explanation on how to escape single quotes in string literal

ADQL.tex

@@ -383,13 +383,21 @@ \subsubsection{Case sensitivity}
 \subsubsection{Literals}
 \label{sec:literals}
 
-String literals are expressed as a character expression delimited by single quotes.
+String literals are expressed as a character expression delimited by single
+quotes.
 
 \begin{verbatim}
     <character_string_literal> ::=
         <quote> [ <character_representation>... ] <quote>
 \end{verbatim}
 
+A single quote inside a string literal must be escaped with a second single
+quote. For instance:
+
+\begin{verbatim}
+    'Earth''s satellite is the Moon.'
+\end{verbatim}
+
 Numeric literals are expressed as an exact decimal value, e.g. \verb:12: or
 \verb:12.3:, or a floating point number with an exponent, e.g. \verb:12.3E4:.
 
@@ -2920,6 +2928,8 @@ \subsection{Between 2.0 and 2.1}
                           (see \AppendixRef{sec:features})
                     \item Syntax description for comments inside a query
                           (see \SectionRef{sec:comments})
+                    \item Description of how to escape single quotes in a string
+                          literal (see \SectionRef{sec:literals})
                 \end{itemize}
             \item \textbf{Updated}
                 \begin{itemize}