Append a text string to the COMMENT keyword's value.
If the record rec has an COMMENT keyword, and if the COMMENT keyword has an existing non-empty-string value, the text string str will be appended to the existing text string. If the COMMENT keyword has no existing text-string value, then str will become the COMMENT keyword value.
If newline is set to 1, then prior to appending str, a newline character will be appended to an existing text-string value. If there is no existing text-string value, then newline has no effect.