Add documentation for parameters of string_from_unterminated

src/helper_functions.hpp

@@ -37,6 +37,13 @@
   Convert a C style string that may or may not be null terminated safely
   into a std::string. The string's termination is either set at the first \0
   or after data_length characters.
+
+  @param[in] data  A c-string from which the std::string shall be
+      constructed. Does not need to be null terminated.
+  @param[in] data_length  An upper bound for the string length (must be at most
+      the allocated length of `buffer`). If no null terminator is found in data,
+      then the resulting std::string will be null terminated at `data_length`.
+
  */
 std::string string_from_unterminated(const char* data, size_t data_length);