feat(guide): Explained the difference between declaring & assigning a constant in Swift (#18515)

Added a code snippet to explain the difference between _declaring_ a constant and _assigning_ a value to it.

client/src/pages/guide/english/swift/constants/index.md

@@ -14,7 +14,16 @@ You declare constants with the `let` keyword then give it a name `name` then you
 
 Once you have declared a constant you don't need to use the `let` keyword anymore you just call it by its name.
 
-The value of a constant can’t be changed once it’s set.
+The value of a constant can’t be changed once it’s _set_. With that being said, it is important to note that the Swift compiler is smart enough to understand the difference between _declaring_ a constant, and _assigning_ a value to it. Consider the following code snippet:
+```swift
+let shouldWaterFreeze: Bool // (1)
+if temperature < 0 {
+  shouldWaterFreeze = true // (2)
+else {
+  shouldWaterFreeze = false // (3)
+}
+```
+The snippet above is valid and compiles with no problems (given that we have declared and assigned an `Int` value to `temparature` somewhere earlier). Notice that we _declare_ the constant in (1), and then _assign_ a value to it (2) or (3).
 
 #### More Information:
 - <a href='https://docs.swift.org/swift-book/LanguageGuide/TheBasics.html#ID310' target='_blank' rel='nofollow'>The Swift Programming Language</a>