Hi Denacthwwacht,
USB is natively 5v no matter the source. USB devices are responsible for stepping down (or up) the voltage to meet their needs.
USB actually uses a clever combination of hardware pins and software negotiation to discover how fast a device can charge. Any properly certified charger that obeys the USB specs should work fine to charge any other compliant USB device. (A reminder that anything I say on this forums should not be considered official advice, our website is the final arbiter and represents our recommended best practices.)
There was a brief time period when quick charging phones first came out when you could find chargers that could potentially cause damage to other manufacturer's devices. Thankfully those chargers are really rare, typically whoever released them got ripped apart in reviews.
You can also come across really cheap knock-off chargers that are not safe to use. If you like electronics, my favorite tear down of an unsafe charger is over at
Ken Shirriff's blog.
The vast majority of name brand Computer USB ports go through a certification process and have a good deal of safety testing done on them. I wasn't involved in the decision of what to write on that page, but I'm guessing that it is was just easiest to say "plug in to your PC" versus trying to explain what a high quality wall wart is.