[char.traits] Better cross-reference several headers

Author: AlisdairM

source/strings.tex

@@ -201,7 +201,7 @@
 \begin{codeblock}
 template<class charT> struct char_traits;
 \end{codeblock}
-is provided in the header \libheader{string}
+is provided in the header \libheaderref{string}
 as a basis for explicit specializations.
 
 \rSec2[char.traits.typedefs]{Traits typedefs}
@@ -261,7 +261,7 @@
 \end{codeblock}
 
 \pnum
-The header \libheader{string}
+The header \libheaderref{string}
 defines five specializations of the class template
 \tcode{char_traits}:
 \tcode{char_traits<\brk{}char>},
@@ -307,7 +307,7 @@
 \end{codeblock}
 
 \pnum
-The type \tcode{mbstate_t} is defined in \libheader{cwchar}
+The type \tcode{mbstate_t} is defined in \libheaderref{cwchar}
 and can represent any of the conversion states that can occur in an
 \impldef{supported multibyte character encoding rules} set of supported multibyte
 character encoding rules.