Clarify QTextTable::mergeCells() documentation

http://qt-project.org/doc/qt-5/qtexttable.html#mergeCells

says: "If numRows or numCols is less than the current number of rows or columns the cell spans then this method does nothing."

it would be more clear if it was:
"This method does nothing if numRows or numCols is less than the current number of rows and columns spanned by the cell."