chore: add doc for the field attribute (#3209)

docs/docs/language_concepts/01_functions.md

@@ -103,3 +103,46 @@ assert(add_50(100) == 150);
 ```
 
 See [Lambdas](./08_lambdas.md) for more details.
+
+## Attributes
+
+Attributes are metadata that can be applied to a function, using the following syntax: `#[attribute(value)]`.
+
+Supported attributes include:
+- **builtin**: the function is implemented by the compiler, for efficiency purposes.
+- **deprecated**: mark the function as *deprecated*. Calling the function will generate a warning: `warning: use of deprecated function`
+- **field**: Used to enable conditional compilation of code depending on the field size. See below for more details
+- **oracle**: mark the function as *oracle*; meaning it is an external unconstrained function, implemented in noir_js. See [Unconstrained](./05_unconstrained.md) and [Noir js](../noir_js/noir_js.md) for more details.
+- **test**: mark the function as unit tests. See [Tests](../nargo/02_testing.md) for more details
+
+### Field Attribute
+The field attribute defines which field the function is compatible for. The function is conditionally compiled, under the condition that the field attribute matches the Noir native field.
+The field can be defined implicitly, by using the name of the elliptic curve usually associated to it - for instance bn254, bls12_381 - or explicitly by using the field (prime) order, in decimal or hexadecimal form.
+As a result, it is possible to define multiple versions of a function with each version specialized for a different field attribute. This can be useful when a function requires different parameters depending on the underlying elliptic curve.
+
+
+Example: we define the function `foo()` three times below. Once for the default Noir bn254 curve, once for the field $\mathbb F_{23}$, which will normally never be used by Noir, and once again for the bls12_381 curve.
+```rust
+#[field(bn254)]
+fn foo() -> u32 {
+    1
+}
+
+#[field(23)]
+fn foo() -> u32 {
+    2
+}
+
+// This commented code would not compile as foo would be defined twice because it is the same field as bn254
+// #[field(21888242871839275222246405745257275088548364400416034343698204186575808495617)]
+// fn foo() -> u32 {
+//     2
+// }
+
+#[field(bls12_381)]
+fn foo() -> u32 {
+    3
+}
+```
+
+If the field name is not known to Noir, it will discard the function. Field names are case insensitive.