Define 'package' in the reference

Author: amartini51

TSPL.docc/ReferenceManual/Declarations.md

@@ -3880,6 +3880,13 @@ Access control is discussed in detail in <doc:AccessControl>.
   Apply this modifier to a declaration
   to indicate that the declaration can be accessed
   only by code in the same package as the declaration.
+  A package is a unit of code distribution
+  that you define in the build system you're using.
+  When the build system compiles code,
+  it specifies the package name
+  by passing the `-package-name` flag to the Swift compiler.
+  Two modules are part of the same package
+  if the build system specifies the same package name when building them both.
 
 - term `internal`:
   Apply this modifier to a declaration to indicate the declaration can be accessed