I guess you refer to that missleading sentence:
The pose library is a way to store pose positions so the same position can be used again later in the animation or to store different rest poses.
TL;DR: The doc tries to say that you can use the pose library to store poses that you can use as default pose. But you never touch the rest pose. Click me for more details.
You can’t create several rest poses. The rest pose in Blender is what is called the “binding pose” in the industry. Its function is to tell your software what are the default bones transforms and how each vertice is located from its influencer bone. It’s a technical notion that is supposed to be dealt with once and for all during the rigging process and never changed afterwards. As changing the rest pose affects every other pose & animation of the rig.
The “binding pose” is often not usable for animation, as it’s usually a T pose or A pose whose only use is to have straightened bones (which is unnatural) as reference while having the vertices not too far from what they will do once animated (which will make the skinning process easier and the deformations more accurate). In fine the “binding pose” is only usefull for the rigging. Not animating.
But, when a character is animated, the animators will usually create at least one “default pose” (usually just the character standing up in a neutral way, or in accordance to his characterization). Which they will start from almost everytime they do an animation. In fact, they will spend a lot of time building an entire library of different poses so they can work quicker aftewards: default stands, crouch, facial shapes, hand shapes, etc.
Unfortunately, the notions of “binding pose” and “default pose” does not exist in Blender. There’s only the “rest pose”, that is technically a “rest pose” considering its technical traits, but sometimes people try to use the rest pose as a “default pose”, leading to some confusion as to what you can/should do with that.
So when you use the pose library, you never touch the rest pose. You just store regular poses with the extra bonus of assigning them a name and not having to use your already cluttured timeline to store them.