chore: Document gate savings around no_predicates

docs/docs/explainers/explainer-writing-noir.md

@@ -184,6 +184,18 @@ Use `  if is_unconstrained() { /`, to conditionally execute code if being called
 
 Unless you're well into the depth of gate optimization, this advanced section can be ignored.
 
+### `#[no_predicates]` over for "pure" functions
+
+When conditional logic is compiled into gates, it is predicated with boolean expressions like X, where `X * V1() + (1-X) * V2()` is calculated. This returns `V1()` when `X` is true or `V2()` if `X` is false.
+
+If a function is guaranteed to never fail an assertion no matter the inputs, or roughly considered "pure", it can use the macro `#[no_predicates]`, telling the compiler to not add such predicates. This can beneficially be used in say hashing functions to reduce the gate count when they intuitively must be written/used inside an `if`.
+
+:::note
+If a function may call other predicated functions, do not mark it with `#[no_predicates]`.
+:::
+
+Having a `#[no_predicates]` function call a regular function severely breaks the intended representation, proof generation, and verification of the program.
+
 ### Combine arithmetic operations
 
 A Noir program can be honed further by combining arithmetic operators in a way that makes the most of each constraint of the backend proving system. This is in scenarios where the backend might not be doing this perfectly.