[2656] Add API doc for CNAME constructors

src/lib/dns/rdata/generic/cname_5.cc

@@ -31,6 +31,20 @@ using isc::dns::rdata::generic::detail::createNameFromLexer;
 // BEGIN_ISC_NAMESPACE
 // BEGIN_RDATA_NAMESPACE
 
+/// \brief Constructor from string.
+///
+/// The given string must represent a valid CNAME RDATA.  There can be extra
+/// space characters at the beginning or end of the text (which are simply
+/// ignored), but other extra text, including a new line, will make the
+/// construction fail with an exception.
+///
+/// The CNAME must be absolute since there's no parameter that specifies
+/// the origin name; if it is not absolute, \c MissingNameOrigin
+/// exception will be thrown.  These must not be represented as a quoted
+/// string.
+///
+/// \throw Others Exception from the Name and RRTTL constructors.
+/// \throw InvalidRdataText Other general syntax errors.
 CNAME::CNAME(const std::string& namestr) :
     // Fill in dummy name and replace it soon below.
     cname_(Name::ROOT_NAME())
@@ -59,6 +73,22 @@ CNAME::CNAME(InputBuffer& buffer, size_t) :
     // check consistency.
 }
 
+/// \brief Constructor with a context of MasterLexer.
+///
+/// The \c lexer should point to the beginning of valid textual
+/// representation of a CNAME RDATA.  The CNAME field can be
+/// non-absolute if \c origin is non-NULL, in which case \c origin is
+/// used to make it absolute.  It must not be represented as a quoted
+/// string.
+///
+/// \throw MasterLexer::LexerError General parsing error such as missing field.
+/// \throw Other Exceptions from the Name and RRTTL constructors if
+/// construction of textual fields as these objects fail.
+///
+/// \param lexer A \c MasterLexer object parsing a master file for the
+/// RDATA to be created
+/// \param origin If non NULL, specifies the origin of CNAME when it
+/// is non-absolute.
 CNAME::CNAME(MasterLexer& lexer, const Name* origin,
              MasterLoader::Options, MasterLoaderCallbacks&) :
     cname_(createNameFromLexer(lexer, origin))