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)

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.

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