Auto merge of #12122 - epage:msrv-docs, r=ehuss

docs(ref): Clarify MSRV is generally minor

For the second time in the last week or two, I've seen the wording of cargo's semver documentation used to justify pushing back against treating MSRV bumps as minor changes.
See time-rs/time#535 (comment).  My hope is that this change will reduce confusion from readers which I hope will reduce conversation fatigue on this topic from maintainers.

The current wording makes it sound like "major" is the default stance but the [general consensus seems to be around
"minor"](rust-lang/api-guidelines#231).  I held back from changing from "possibly major" to minor" for now as the above linked policy isn't official yet and it leaves it open for crates to treat it as major.

I tried to keep the scope of this change small to reduce risk of the discussion being slowed down for specific points unrelated to my main goal from delaying the improvement I'm trying to make, the idea that "some improvement is better than no improvement".

Fixes #12121

src/doc/src/reference/semver.md

@@ -1251,7 +1251,8 @@ projects that are using older versions of Rust. This also includes using new
 features in a new release of Cargo, and requiring the use of a nightly-only
 feature in a crate that previously worked on stable.
 
-Some projects choose to allow this in a minor release for various reasons. It
+It is generally recommended to treat this as a minor change, rather than as
+a major change, for [various reasons][msrv-is-minor]. It
 is usually relatively easy to update to a newer version of Rust. Rust also has
 a rapid 6-week release cycle, and some projects will provide compatibility
 within a window of releases (such as the current stable release plus N
@@ -1530,3 +1531,4 @@ document what your commitments are.
 [struct literal]: ../../reference/expressions/struct-expr.html
 [wildcard patterns]: ../../reference/patterns.html#wildcard-pattern
 [unused_unsafe]: ../../rustc/lints/listing/warn-by-default.html#unused-unsafe
+[msrv-is-minor]: https://github.com/rust-lang/api-guidelines/discussions/231