gsu: Improve documentation of gsu_getopts().

gsu: Improve documentation of gsu_getopts().

The sentence which explains how $o_X is set for both types of options
was rather confusing. The new text should be better.

While we're at it,
	* mention that getopts is a bash builtin
	* switch to "modern" ((expr)) arithmetic evaluation in example
	* use '' to quote constants in example