docs: Add doc for InitSpace macro (#2521)

docs/src/pages/docs/space.md

@@ -12,6 +12,8 @@ so there the `C` layout applies.
 
 In addition to the space for the account data, you have to add `8` to the `space` constraint for Anchor's internal discriminator (see the example).
 
+## Type chart
+
 | Types      | Space in bytes                | Details/Example                                                                                 |
 | ---------- | ----------------------------- | ----------------------------------------------------------------------------------------------- |
 | bool       | 1                             | would only require 1 bit but still uses 1 byte                                                  |
@@ -29,7 +31,7 @@ In addition to the space for the account data, you have to add `8` to the `space
 | f32        | 4                             | serialization will fail for NaN                                                                 |
 | f64        | 8                             | serialization will fail for NaN                                                                 |
 
-# Example
+## Example
 
 ```rust
 #[account]
@@ -59,3 +61,33 @@ pub struct InitializeMyData<'info> {
     pub system_program: Program<'info, System>
 }
 ```
+
+## The InitSpace macro
+
+Sometimes it can be difficult to calculate the initial space of an account. This macro will add an `INIT_SPACE` constant to the structure. It is not necessary for the structure to contain the `#[account]` macro to generate the constant. Here's an example:
+
+```rust
+#[account]
+#[derive(InitSpace)]
+pub struct ExampleAccount {
+    pub data: u64,
+    #[max_len(50)]
+    pub string_one: String,
+    #[max_len(10, 5)]
+    pub nested: Vec<Vec<u8>>,
+}
+
+#[derive(Accounts)]
+pub struct Initialize<'info> {
+    #[account(mut)]
+    pub payer: Signer<'info>,
+    pub system_program: Program<'info, System>,
+    #[account(init, payer = payer, space = 8 + ExampleAccount::INIT_SPACE)]
+    pub data: Account<'info, ExampleAccount>,
+}
+```
+
+A few important things to know:
+
+- Don't forget the discriminator when defining `space`
+- The `max_len` length represents the length of the structure, not the total length. (ie: the `max_len` of a Vec<u32> will be `max_len` \* 4)

lang/derive/accounts/src/lib.rs

@@ -117,7 +117,7 @@ use syn::parse_macro_input;
 ///                         The given space number is the size of the account in bytes, so accounts that hold
 ///                         a variable number of items such as a <code>Vec</code> should allocate sufficient space for all items that may
 ///                         be added to the data structure because account size is fixed.
-///                         Check out the <a href = "https://book.anchor-lang.com/anchor_references/space.html" target = "_blank" rel = "noopener noreferrer">space reference</a>
+///                         Check out the <a href = "https://www.anchor-lang.com/docs/space" target = "_blank" rel = "noopener noreferrer">space reference</a>
 ///                         and the <a href = "https://borsh.io/" target = "_blank" rel = "noopener noreferrer">borsh library</a>
 ///                         (which anchor uses under the hood for serialization) specification to learn how much
 ///                         space different data structures require.