Update docs

Author: luispadron

doc/providers.md

@@ -48,6 +48,32 @@ where the arguments are:
 When reading an existing `SwiftInfo` provider, it has the two fields described
 below.
 
+This provider has a custom initializer that will merge the transitive modules of
+a list of `SwiftInfo` providers, rather than a separate "merge" function. The
+correct signature when _creating_ a new `SwiftInfo` provider is the following:
+
+```build
+SwiftInfo(
+    direct_swift_infos,
+    modules,
+    swift_infos,
+)
+```
+
+where the arguments are:
+
+*   `direct_swift_infos`: A list of `SwiftInfo` providers from dependencies
+    whose direct modules should be treated as direct modules in the resulting
+    provider, in addition to their transitive modules being merged.
+*   `modules`: A list of values (as returned by `create_swift_module_context`)
+    that represent Clang and/or Swift module artifacts that are direct outputs
+    of the target being built.
+*   `swift_infos`: A list of `SwiftInfo` providers from dependencies whose
+    transitive modules should be merged into the resulting provider.
+
+When reading an existing `SwiftInfo` provider, it has the two fields described
+below.
+
 **FIELDS**