Fix bad method links in docstrings (#13471)

# Description

Seems like I developed a bit of a bad habit of trying to link

```rust
/// [`.foo()`]
```

in docstrings, and this just doesn't work automatically; you have to do 

```rust
/// [`.foo()`](Self::foo)
```

if you want it to actually link. I think I found and replaced all of
these.

# User-Facing Changes

Just docs.

crates/nu-protocol/src/engine/engine_state.rs

@@ -832,9 +832,9 @@ impl EngineState {
 
     /// Optionally get a block by id, if it exists
     ///
-    /// Prefer to use [`.get_block()`] in most cases - `BlockId`s that don't exist are normally a
-    /// compiler error. This only exists to stop plugins from crashing the engine if they send us
-    /// something invalid.
+    /// Prefer to use [`.get_block()`](Self::get_block) in most cases - `BlockId`s that don't exist
+    /// are normally a compiler error. This only exists to stop plugins from crashing the engine if
+    /// they send us something invalid.
     pub fn try_get_block(&self, block_id: BlockId) -> Option<&Arc<Block>> {
         self.blocks.get(block_id)
     }

crates/nu-protocol/src/engine/stack.rs

@@ -51,7 +51,7 @@ pub struct Stack {
     pub parent_stack: Option<Arc<Stack>>,
     /// Variables that have been deleted (this is used to hide values from parent stack lookups)
     pub parent_deletions: Vec<VarId>,
-    /// Locally updated config. Use [`.get_config()`] to access correctly.
+    /// Locally updated config. Use [`.get_config()`](Self::get_config) to access correctly.
     pub config: Option<Arc<Config>>,
     pub(crate) out_dest: StackOutDest,
 }