Start sketching reference for @preconcurrency

Author: amartini51

TSPL.docc/ReferenceManual/Attributes.md

@@ -1304,6 +1304,54 @@ can increase your binary size and adversely affect performance.
   because of the larger symbol table slowing dyld down.
 -->
 
+### preconcurrency
+
+Apply this attribute to an import declaration or a declaration,
+to suppress strict concurrency checking.
+
+On an import declaration,
+this attribute reduces the strictness of conccurrency checking
+for code that uses types from the imported module.
+Specifically,
+types fromthe imported module
+that aren't explicitly marked as either sendeble or nonsendable
+can be used in a context that requires sendable types.
+<!--XXX If the attribute isn't doing anything, it produces a warning.-->
+
+You can apply this attribute
+to the following kinds of declarations:
+
+- Structures, classes, and actors
+- Enumerations and enumeration cases
+- Protocols
+- Variables and constants
+- Subscripts
+- Initializers
+- Functions
+
+On a declaration,
+this attribute indicates
+
+<!-- XXX from SE-0337
+Its name is mangled as though it does not use any of the listed features.
+
+At use sites whose enclosing scope uses Minimal concurrency checking, the compiler will suppress any diagnostics about mismatches in these traits.
+
+The ABI checker will remove any use of these features when it produces its digests.
+-->
+
+<!-- Probably move to some migration article/context:
+
+You can use this attribute to aid in migration to strict concurrency checking:
+
+1. Enable strict checking
+1. Annotate imports with `@preconcurrency` as needed
+1. After migrating the imported module, XXX
+1. Remove `@preconcurrency` from import declarations
+-->
+
+Declarations from Objective-C are always imported
+as if they were marked with the `preconcurrency` attribute.
 ### propertyWrapper
 
 Apply this attribute to a class, structure, or enumeration declaration