Add stable documentation for the [hints] table

Author: joshtriplett

src/doc/src/reference/manifest.md

@@ -50,6 +50,7 @@ Every manifest file consists of the following sections:
 * [`[badges]`](#the-badges-section) --- Badges to display on a registry.
 * [`[features]`](features.md) --- Conditional compilation features.
 * [`[lints]`](#the-lints-section) --- Configure linters for this package.
+* [`[hints]`](#the-hints-section) --- Provide hints for compiling this package.
 * [`[patch]`](overriding-dependencies.md#the-patch-section) --- Override dependencies.
 * [`[replace]`](overriding-dependencies.md#the-replace-section) --- Override dependencies (deprecated).
 * [`[profile]`](profiles.md) --- Compiler settings and optimizations.
@@ -565,6 +566,21 @@ As for dependents, Cargo suppresses lints from non-path dependencies with featur
 
 > **MSRV:** Respected as of 1.74
 
+## The `[hints]` section
+
+The `[hints]` section allows specifying hints for compiling this crate, which
+crates depending on this one will use by default but may override. Hints are,
+by design, always safe for Cargo to ignore; if Cargo encounters a hint it
+doesn't understand, or a hint it understands but with a value it doesn't
+understand, it will warn, but not error.
+
+Individual hints may have an associated unstable feature gate that you need to
+pass in order to apply the configuration they specify, but if you don't specify
+that unstable feature gate, you will again get only a warning, not an error.
+
+> **MSRV:** Specifying hints does not impact MSRV. Older versions of Cargo will
+> ignore hints with a warning, but will not error.
+
 ## The `[badges]` section
 
 The `[badges]` section is for specifying status badges that can be displayed