Update git2 API mapping and be clear what stability means as well.

STABILITY.md

@@ -96,6 +96,8 @@ It's acceptable to let each breaking change be immediately followed by a minor v
 
 Released plumbing crates are marked with major version number 1 or above, for example `1.2.4` and live in stability tier 2 _(->ST2)_.
 
+*These crates may not expose parts of unstable crates in their public API*.
+
 Breaking changes are collected and may be released no more often than every 4 weeks by incrementing the major version number.
 
 For example, `gix-odb` and `gix-ref` both have breaking changes, where `gix-odb`'s change is on August 1st and `gix-ref`'s dependent change

gix/src/lib.rs

@@ -66,6 +66,7 @@
 //! * [`git2::Repository::revparse_single()`](https://docs.rs/git2/*/git2/struct.Repository.html#method.revparse_single) ➡ [`gix::Repository::rev_parse_single()`](https://docs.rs/gix/*/gix/struct.Repository.html#method.rev_parse_single)
 //! * [`git2::Repository::revwalk()`](https://docs.rs/git2/*/git2/struct.Repository.html#method.revwalk) ➡ [`gix::Repository::rev_walk()`](https://docs.rs/gix/*/gix/struct.Repository.html#method.rev_walk)
 //! * [`git2::build::CheckoutBuilder::disable_filters()](https://docs.rs/git2/0.17.2/git2/build/struct.CheckoutBuilder.html#method.disable_filters) ➡ ❌ *(filters are always applied during checkouts)*
+//! * [`git2::Odb::read_header()`](https://docs.rs/git2/0.17.2/git2/struct.Odb.html#method.read_header) ➡ [`gix::Repository::find_header()`](https://docs.rs/gix/*/gix/struct.Repository.html#method.find_header)
 //!
 //! ## Feature Flags
 #![cfg_attr(