docs: Document using 'return' string in expressions (#3625)

Author: claremacrae

docs/Scripting/Expressions.md

@@ -86,6 +86,8 @@ As of Tasks 4.4.0, it is also possible to use more complex constructs in express
 - `if` statements
 - functions
 
+Note in the examples below that if `return` appears anywhere in the expression, you must write your own `return` statement, to prevent the result being `undefined`.
+
 As of Tasks 5.0.0, it is also possible to make longer expressions more readable using [[Line Continuations|line continuations]].
 
 <!-- placeholder to force blank line before included text --><!-- include: Expression.test.Expression_returns_and_functions.approved.md -->
@@ -97,6 +99,12 @@ return 42
 const x = 1 + 1; return x * x
 => 4
 
+'any text that contains the word return'
+=> undefined
+
+return 'any text that contains the word return'
+=> 'any text that contains the word return'
+
 if (1 === 1) { return "yes"; } else { return "no" }
 => 'yes'