Improve docs on semver 'Working With Prerelease Versions' section

[BdN3504]

The phrase

That means >=1.2.3-BETA will return 1.2.3-alpha

in the Working With Prerelease Versions section is nonsensical. There should be a concrete example for this behavior. It seems the documentation in that section is copied directly from the semver repository. Maybe it would be better to just link to those docs rather than use an unaltered copy of them in the helm docs.