-columns, counting from 0 at the left margin. The column position is the
-sum of the widths of all the displayed representations of the characters
-between the start of the current line and point.
+columns, counting from 0 at the left margin.
+
+This is calculated by adding together the widths of all the displayed
+representations of the character between the start of the previous line
+and point. (e.g. control characters will have a width of 2 or 4, tabs
+will have a variable width.)
+
+Ignores the finite width of frame displaying the buffer, which means
+that this function may return values greater than
+@code{(frame-width)}.
+
+Whether the line is visible (if @code{selective-display} is t) has no effect;
+however, ^M is treated as end of line when @code{selective-display} is t.
+
+If @var{buffer} is nil, the current buffer is assumed.