Update discussion of macros as default values [SE-0422] (#328)

Fixes: rdar://133387175

Author: amartini51

TSPL.docc/ReferenceManual/Expressions.md

@@ -1520,14 +1520,39 @@ Macro-expansion expressions have the following form:
 A macro-expansion expression omits the parentheses after the macro's name
 if the macro doesn't take any arguments.
 
-A macro-expansion expression can't appear as the default value for a parameter,
-except the [`file()`][] and [`line()`][] macros from the Swift standard library.
+A macro-expansion expression can appear as the default value for a parameter.
 When used as the default value of a function or method parameter,
-these macros are evaluated using the source code location of the call site,
+macros are evaluated using the source code location of the call site,
 not the location where they appear in a function definition.
+However, when a default value is a larger expression
+that contains a macro in addition to other code,
+those macros are evaluated where they appear in the function definition.
 
-[`file()`]: https://developer.apple.com/documentation/swift/file()
-[`line()`]: https://developer.apple.com/documentation/swift/line()
+```
+func f(a: Int = #line, b: Int = (#line), c: Int = 100 + #line) {
+    print(a, b, c)
+}
+f()  // Prints "4 1 101"
+```
+
+In the function above,
+the default value for `a` is a single macro expression,
+so that macro is evaluated using the source code location
+where `f(a:b:c:)` is called.
+In contrast, the values for `b` and `c`
+are expressions that contain a macro ---
+the macros in those expressions are evaluated
+using the source code location where `f(a:b:c:)` is defined.
+
+When you use a macro as a default value,
+it's type checked without expanding the macro,
+to check the following requirements:
+
+- The macro's access level
+  is the same as or less restrictive than the function that uses it.
+- The macro either takes no arguments,
+  or its arguments are literals without string interpolation.
+- The macro's return type matches the parameter's type.
 
 You use macro expressions to call freestanding macros.
 To call an attached macro,