[lex.string] Remove unnecessary linebreaks and use bnf instead of bnftab.

Use \bnfindent for indentation in the few remaining cases where it is required.

Author: tkoeppe

source/lex.tex

@@ -1424,26 +1424,24 @@
     r-char-sequence r-char
 \end{bnf}
 
-\begin{bnftab}
+\begin{bnf}
 \nontermdef{r-char}\br
-\>\textnormal{any member of the source character set, except}\br
-\>\>\textnormal{a right parenthesis \terminal{)} followed by the initial \grammarterm{d-char-sequence}}\br
-\>\>\textnormal{(which may be empty) followed by a double quote \terminal{"}.}
-\end{bnftab}
+    \textnormal{any member of the source character set, except a right parenthesis \terminal{)} followed by}\br
+    \bnfindent\textnormal{the initial \grammarterm{d-char-sequence} (which may be empty) followed by a double quote \terminal{"}.}
+\end{bnf}
 
 \begin{bnf}
 \nontermdef{d-char-sequence}\br
     d-char\br
     d-char-sequence d-char
 \end{bnf}
 
-\begin{bnftab}
+\begin{bnf}
 \nontermdef{d-char}\br
-\>\textnormal{any member of the basic source character set except:}\br
-\>\>\textnormal{space, the left parenthesis \terminal{(}, the right parenthesis \terminal{)}, the backslash \terminal{\textbackslash},}\br
-\>\>\textnormal{and the control characters representing horizontal tab,}\br
-\>\>\textnormal{vertical tab, form feed, and newline.}
-\end{bnftab}
+    \textnormal{any member of the basic source character set except:}\br
+    \bnfindent\textnormal{space, the left parenthesis \terminal{(}, the right parenthesis \terminal{)}, the backslash \terminal{\textbackslash}, and the control characters}\br
+    \bnfindent\textnormal{representing horizontal tab, vertical tab, form feed, and newline.}
+\end{bnf}
 
 \pnum
 \indextext{literal!string}%