Use better example for lazy eval and explicitly explain that the value won't be recomputed each time

Author: MasterNayru

README.md

@@ -1497,11 +1497,27 @@ variable to only be evaluated when it is used for the first time, you can use th
 `lazy` keyword:
 
 ```just
-lazy password := `aws ecr get-login-password --region us-west-2`
+lazy aws_account_id := `aws sts get-caller-identity --query Account --output text`
+```
+
+Once a lazy variable has been evaluated, its value is the same for the rest of
+the invocation of `just`, even if it is used multiple times:
+
+```just
+lazy timestamp := `date +%s`
+
+foo:
+  # The value is computed here
+  echo The time is {{timestamp}}
+  sleep 1
+  # The same value is used here
+  echo The time is still {{timestamp}}
 ```
 
 This is useful for values that are expensive to compute, or that may not be
-needed in every invocation of `just`.
+needed in every invocation of `just`. It also saves you from having expensive
+values being recomputed even for simple invocations of `just` that don't
+actually use them, like `just --list`.
 
 ### Strings