Document that swiftpmConfig.name must match the manifest's product

The aggregator depends on a self-managed library as
.product(name: "<SwiftName>", package: "<SwiftName>"), so the library's
Package.swift has to declare a product with that name. The rule holds
for a manifest at the root, in ios/ or in apple/.

Author: mrousavy

packages/react-native/scripts/spm/__docs__/spm-scripts.md

@@ -613,6 +613,12 @@ then in `ios/`, then in `apple/`, and uses the first one it did not generate
 itself. A manifest in `ios/` or `apple/` keeps SwiftPM artifacts (`.build/`,
 `.swiftpm/`, `Package.resolved`) out of the npm package root.
 
+For a self-managed library, `swiftpmConfig.name` must equal the name of a
+product its `Package.swift` declares. The aggregator depends on the library as
+`.product(name: "<SwiftName>", package: "<SwiftName>")`, and SwiftPM fails to
+resolve the package graph if no product has that name. A manifest in `ios/` or
+`apple/` follows the same rule as one at the root.
+
 SwiftPM derives package identity from the path basename, and several libs may
 put their manifest inside an `ios/` or `apple/` subdir. To keep identity unique,
 each self-managed dep is exposed through a uniquely-named symlink at