Incorrect references to SemVer

[lann]

We've been a little loose in some references to SemVer, especially as it relates to compatibility rules. The SemVer spec itself only defines compatibility for major version > 0 (i.e. >=1.0.0), but the WIT documentation for @since (e.g. here) implies a compatibility requirement between e.g. 0.1.0 and 0.1.1. This does match the popular "caret requirements" versioning strategy used by default in cargo, npm, and elsewhere. We should just be more explicit that this isn't "plain" SemVer.