update_extra_cmt

Set or update extra comment line
     ea   - linear address
     n    - number of additional line (0..MAX_ITEM_LINES)
     line - the line to display
IDA displays additional lines from number 0 up to the first unexisting
additional line. So, if you specify additional line #150 and there is no
additional line #149, your line will not be displayed.
MAX_ITEM_LINES is defined in IDA.CFG
To set anterior  line #n use (E_PREV + n)
To set posterior line #n use (E_NEXT + n)

void update_extra_cmt(long ea, long n, string line);