Misc doc fixes (#12266)

# Description

Just a bunch of miscellaneous fixes to the Rust documentation that I
found recently while doing
a pass on some things.

# User-Facing Changes
None

crates/nu-protocol/src/pipeline_data/io_stream.rs

@@ -9,7 +9,8 @@ pub enum IoStream {
     /// If both `stdout` and `stderr` are set to `Pipe`,
     /// then they will combined into `ExternalStream::stdout`.
     Pipe,
-    /// Capture output to later be collected into a [`Value`], `Vec`, or used in some other way.
+    /// Capture output to later be collected into a [`Value`](crate::Value), `Vec`, or used in some
+    /// other way.
     ///
     /// The output stream(s) will be available in
     /// `PipelineData::ExternalStream::stdout` or `PipelineData::ExternalStream::stderr`.

crates/nu-protocol/src/pipeline_data/mod.rs

@@ -876,7 +876,7 @@ impl PipelineData {
 
     /// Consume and print self data immediately.
     ///
-    /// Unlike [print] does not call `table` to format data and just prints it
+    /// Unlike [`.print()`] does not call `table` to format data and just prints it
     /// one element on a line
     /// * `no_newline` controls if we need to attach newline character to output.
     /// * `to_stderr` controls if data is output to stderr, when the value is false, the data is output to stdout.

crates/nu-protocol/src/plugin/registered.rs

@@ -2,7 +2,7 @@ use std::{any::Any, sync::Arc};
 
 use crate::{PluginGcConfig, PluginIdentity, ShellError};
 
-/// Trait for plugins registered in the [`EngineState`](crate::EngineState).
+/// Trait for plugins registered in the [`EngineState`](crate::engine::EngineState).
 pub trait RegisteredPlugin: Send + Sync {
     /// The identity of the plugin - its filename, shell, and friendly name.
     fn identity(&self) -> &PluginIdentity;

crates/nu-protocol/src/value/custom_value.rs

@@ -80,8 +80,7 @@ pub trait CustomValue: fmt::Debug + Send + Sync {
     /// For custom values in plugins: return `true` here if you would like to be notified when all
     /// copies of this custom value are dropped in the engine.
     ///
-    /// The notification will take place via
-    /// [`.custom_value_dropped()`](crate::Plugin::custom_value_dropped) on the plugin.
+    /// The notification will take place via `custom_value_dropped()` on the plugin type.
     ///
     /// The default is `false`.
     fn notify_plugin_on_drop(&self) -> bool {