indent: clean up verbose documentation comments from the 1970s

Since C90, there is no need to repeat the type of the function
parameters.

In the whole code of indent, there is a lot of confusion between the
concepts of a 'column' (which is a position on the screen, counting
starts at 1) and 'indentation' (which is a length, not a position).  To
avoid this confusion, the code will be rewritten anyway very soon.

Repeatedly adding and subtracting 1 from the 'current column' is not
elegant, this should rather be done by consistently measuring only the
indentation from the left border (at offset 0), as a distance, not as an
absolute position.

(rillig)