Initializes iter to a position char_offset chars from the start of the entire buffer.
If char_offset is -1 or greater than the number of characters in the buffer, iter is initialized to the end
iterator, the iterator one past the last valid character in the buffer.
| this |
a `GtkTextBuffer` |
| iter |
iterator to initialize |
| char_offset |
char offset from start of buffer, counting from 0, or -1 |