Mention 'open' in discussion of inlinable code

No changes needed here specifically for 'package' access level.

Author: amartini51

TSPL.docc/ReferenceManual/Attributes.md

@@ -937,7 +937,7 @@ The compiler is allowed to replace calls to an inlinable symbol
 with a copy of the symbol's implementation at the call site.
 
 Inlinable code
-can interact with `public` symbols declared in any module,
+can interact with `open` and `public` symbols declared in any module,
 and it can interact with `internal` symbols
 declared in the same module
 that are marked with the `usableFromInline` attribute.