Fix doxygen warnings.

sql/field.h:
  Fix doxygen warnings - too complex return value type, it choked on it.
sql/field_conv.cc:
  Fix doxygen warnings - too complex return value type, it choked on it.

include/mysql/plugin.h

@@ -773,7 +773,7 @@ void thd_get_xid(const MYSQL_THD thd, MYSQL_XID *xid);
   Invalidate the query cache for a given table.
 
   @param thd         user thread connection handle
-  @param key         databasename\0tablename\0
+  @param key         databasename\\0tablename\\0
   @param key_length  length of key in bytes, including the NUL bytes
   @param using_trx   flag: TRUE if using transactions, FALSE otherwise
 */

sql/field.h

@@ -1732,7 +1732,12 @@ class Send_field {
 */
 
 class Copy_field :public Sql_alloc {
-  void (*get_copy_func(Field *to,Field *from))(Copy_field *);
+  /**
+    Convenience definition of a copy function returned by
+    get_copy_func.
+  */
+  typedef void Copy_func(Copy_field*);
+  Copy_func *get_copy_func(Field *to, Field *from);
 public:
   uchar *from_ptr,*to_ptr;
   uchar *from_null_ptr,*to_null_ptr;

sql/field_conv.cc

@@ -620,7 +620,8 @@ void Copy_field::set(Field *to,Field *from,bool save)
 }
 
 
-void (*Copy_field::get_copy_func(Field *to,Field *from))(Copy_field*)
+Copy_field::Copy_func *
+Copy_field::get_copy_func(Field *to,Field *from)
 {
   bool compatible_db_low_byte_first= (to->table->s->db_low_byte_first ==
                                      from->table->s->db_low_byte_first);

sql/log_event.h

@@ -435,7 +435,7 @@ struct sql_ex_info
   ((ULL(1) << 14) | (ULL(1) << 26) | (ULL(1) << 27) | (ULL(1) << 19))
 
 #if OPTIONS_WRITTEN_TO_BIN_LOG != EXPECTED_OPTIONS
-#error 1
+#error
 #endif
 #undef EXPECTED_OPTIONS         /* You shouldn't use this one */
 
@@ -2549,7 +2549,7 @@ class Delete_rows_log_event : public Rows_log_event
    <caption>Incident event format</caption>
    <tr>
      <th>Symbol</th>
-     <th>Size<br/>(bytes)</th>
+     <th>Size<br>(bytes)</th>
      <th>Description</th>
    </tr>
    <tr>