This is, unfortunately, true of the printed so-called "manuals" that ship with most consumer modems these days.

However, you should still be able to find a full description of the supported AT commands by visiting the manufacturer's website, and/or contacting their support department.

An Internet search using the make & model should also help.

Note also that there are actually very few different modem chipsets around these days - if you can identify the chipset in your particular modem, that & an internet search should quickly lead you to full technical specs...